PREFACEThe eighth edition is a major revision in which and water flow is discussed as a function of thethere has been careful revision of the topics hydraulic gradient and conductivity. Darcys Lawcovered as well as changes in the depth of cover- is used in Chapter 6, "Soil Water Management,"age. Many new figures and tables are included. i n regard to water movement in infiltration, drain-Summary statements are given at the ends of the age, and irrigation. Chapter 6 also covers dis-more difficult sections within chapters, and a posal of sewage effluent in soils and prescriptionsummary appears at the end of each chapter. athletic turf (PAT) as an example of precisionMany nonagricultural examples are included to control of the water, air, and salt relationships inemphasize the importance of soil properties when soils used for plant growth. "Soil Erosion," Chap-soils are used in engineering and urban settings. ter 7, has been slightly reorganized with greaterThe topics relating to environmental quality are emphasis on water and wind erosion processes.found throughout the book to add interest to many Chapters 8 and 9, "Soil Ecology" and "Soil Or-chapters. Several examples of computer applica- ganic Matter," are complimentary chapters relat-tion are included. i ng to the biological aspects of soils. The kinds The original Chapter 1, "Concepts of Soil," was and nature of soil organisms and nutrient cyclingsplit into two chapters. Each chapter emphasizes remain as the central themes of Chapter 8. Anan important concept of soil-soil as a medium expanded section on the rhizosphere has beenfor plant growth and soil as a natural body. Topics i ncluded. The distinctions between labile and sta-covered in Chapter 1 include the factors affecting ble organic matter and the interaction of organicplant growth, root growth and distribution, nutri- matter with the minerals (especially clays) areent availability (including the roles of root inter- central themes of Chapter 9. Also, the concept ofception, mass flow and diffusion), and soil fertil- cation exchange capacity is minimally developedi ty and productivity. The importance of soils as a i n the coverage of the nature of soil organic mattersource of nutrients and water is stressed in Chap- i n Chapter 9.ter 1 and elsewhere throughout the book. Chapter Chapter 10, "Soil Mineralogy," and Chapter 11,2 covers the basic soil formation processes of "Soil Chemistry", are complimentary chapters re-humification of organic matter, mineral weather- l ating to the mineralogical and chemical proper-i ng, leaching, and translocation of colloids. The ties of soils. The evolution theme included ini mportant theme is soil as a three-dimensional Chapter 2 is used to develop the concept ofbody that is dynamic and ever-changing. The con- changing mineralogical and chemical propertiescepts developed in the first two chapters are used with time. Soils are characterized as being mini-repeatedly throughout the book. mally, moderately, and intensively weathered, The next five chapters relate to soil physical and these distinctions are used in discussions ofproperties and water. The material on tillage and soil pH, liming, soil fertility and fertilizer use, soiltraffic was expanded to reflect the increasing ef- genesis, and land use.fect of tillage and traffic on soils and plant growth Chapters 12 through 15 are concerned with theand is considered in Chapter 4. The nature of soil general area of soil fertility and fertilizer use.water is presented as a continuum of soil water Chapters 12 and 13 cover the macronutrients andpotentials in Chapter 5. Darcys law is developed micronutrients plus toxic elements, respectively. V
vi PREFACEChapters 14 and 15 cover the nature of fertilizers taxonomy is covered in Chapter 17. This allows aand the evaluation of soil fertility and the use of consideration of soil classification after soilfertilizers, respectively. Greater stress has been properties have been covered. This arrangementplaced on mass flow and diffusion in regard to also makes the book more desirable for use innutrient uptake. The interaction of water and soil two-year agricultural technology programs andfertility is developed, and there is expanded cov- overseas, in countries where Soil Taxonomy is noterage of soil fertility evaluation and the methods used.used to formulate fertilizer recommendations. The final chapter, "Land and the World FoodRecognition is made of the increasing frequency Supply," includes a section on the world grainof high soil test results and the implications for trade and examines the importance of nonagro-fertilizer use and environmental quality. Greater nomic factors in the food-population problem.coverage is given to animal manure as both a Both English and metric units are used in thesource of nutrients and a source of energy. Infor- measurement of crop yields, and for some othermation on land application of sewage sludge and parameters. Using both kinds of units should sat-on sustainable agriculture has been added. isfy both United States and foreign readers.Throughout these four chapters there is a greater Special thanks to Mary Foth for the artwork andemphasis on the importance of soil fertility and to my late son-in-law, Nate Rufe, for photographicfertilizers and on the environmental aspects of contributions. Over the years, many colleaguesgrowing crops. have responded to my queries to expand my The next four chapters (Chapters 16, 17, 18, and knowledge and understanding. Others have pro-19) relate to the areas of soil genesis, soil taxon- vided photographs. The reviewers also have pro-omy, soil geography and land use, and soil survey vided an invaluable service. To these persons, Iand land use interpretations. In this edition, the am grateful.subjects of soil taxonomy (classification) and of Finally, this book is a STORY about soil. Thesoil survey and land use interpretations have re- story reflects my love of the soil and my devotionceived increased coverage in two small chapters. to promoting the learning and understanding ofThe emphasis in the soil geography and land use soils for more than 40 years. I hope that all whochapter is at the suborder level. References to read this book will find it interesting as well asl ower categories are few. Color photographs of i nformative.soil profiles are shown in Color Plates 5 and 6. No Henry D. Fothreference to Soil Taxonomy (USDA) is made until East Lansing, Michigan
BRIEF CONTENTS VII
DETAILED CONTENTSCHAPTER 1 CHAPTER 3SOIL AS A MEDIUM FOR SOIL PHYSICAL PROPERTIES 22PLANT GROWTH 1 SOIL TEXTURE 22FACTORS OF PLANT GROWTH 1 The Soil Separates 22 Particle Size Analysis 24Support for Plants 1 Soil Textural Classes 25Essential Nutrient Elements 2 Determining Texture by the Field Method 25Water Requirement of Plants 3 Influence of Coarse Fregments onOxygen Requirement of Plants 4 Class Names 26Freedom from Inhibitory Factors 5 Texture and the Use of Soils 26PLANT ROOTS AND SOIL RELATIONS 5 SOIL STRUCTURE 27Development of Roots in Soils 5 I mportance of Structure 28Extensiveness of Roots in Soils 7 Genesis and Types of Structure 28Extent of Root and Soil Contact 8 Grade and Class 29Roles of Root Interception, Mass Flow,and Diffusion 8 Managing Soil Structure 29SOIL FERTILITY AND SOIL PRODUCTIVITY 9 SOIL CONSISTENCE 31 Soil Consistence Terms 31 DENSITY AND WEIGHT RELATIONSHIPS 32 Particle Density and Bulk Density 32 Weight of a Furrow-Slice of Soil 33 Soil Weight on a Hectare Basis 34CHAPTER 2 SOIL PORE SPACE AND POROSITY 34SOIL AS A NATURAL BODY 11 Determination of Porosity 34 Effects of Texture and Structure on Porosity 35THE PARENT MATERIAL OF SOIL 12 Porosity and Soil Aeration 35Bedrock Weathering and Formation ofParent Material 12 SOIL COLOR 36Sediment Parent Materials 13 Determination of Soil Color 37 Factors Affecting Soil Color 37SOIL FORMATION 13 Significance of Soil Color 37Soil-Forming Processes 14Formation of A and C Ho zons ri 14 SOIL TEMPERATURE 38Formation of B Horizons 14 Heat Balance of Soils 38 The Bt Horizon 15 Location and Temperature 39 The Bhs Horizon 17 Control of Soil Temperature 39 Permafrost 40Formation of E Horizons 17Formation of 0 Horizons 18 CHAPTER 4SOILS AS NATURAL BODIES 18 TILLAGE AND TRAFFIC 42The Soil-Forming Factors 18Soil Bodies as Parts of Landscapes 19 EFFECTS OF TILLAGE ON SOILS ANDHow Scientists Study Soils as Natural Bodies 19 PLANT GROWTH 42I mportance of Concept of Soil as Natural Body 20 Management of Crop Residues 42 ix
X DETAILED CONTENTS CHAPTER 6 SOIL WATER MANAGEMENT 73 Tillage and Weed Control 43 Effects of Tillage on Structure and Porosity 43 Surface Soil Crusts 44 Minimum and Zero Tillage Concepts 44 WATER CONSERVATION 73 Tilth and Tillage 45 Modifying the Infiltration Rate 73 Summer Fallowing 75 TRAFFIC AND SOIL COMPACTION 46 Saline Seep Due to Fallowing 76 Compaction Layers 46 Effect of Fertilizers on Water Use Efficiency 77 Effects of Wheel Traffic on Soils and Crops 47 Effects of Recreational Traffic 47 SOIL DRAINAGE 78 Effects of Logging Traffic on Soils and Water Table Depth Versus Air and Water Content Tree Growth 48 of Soil 79 Controlled Traffic 49 Benefits of Drainage 80 Surface Drainage 80 FLOODING AND PUDDLING OF SOIL 50 Subsurface Drainage 80 Effects of Flooding 50 Drainage in Soil of Container-Grown Plants 81 Effects of Puddling 50 Oxygen Relationships in Flooded Soils 51 I RRIGATION 82 Water Sources 82 I mportant Properties of Irrigated Soils 82 ,/ Water Application Methods 83 CHAPTER 5 Flood Irrigation 83 SOIL WATER 54 Furrow Irrigation 83 Sprinkler Irrigation 83 SOIL WATER ENERGY CONTINUUM 54 Subsurface Irrigation 85 Drip Irrigation 85 Adhesion Water 55 Cohesion Water 55 Rate and Timing of Irrigation 85 Gravitational Water 56 Water Quality 86 Summary Statements 56 Total Salt Concentration 86 Sodium Adsorption Ratio 86 ENERGY AND PRESSURE RELATIONSHIPS 57 Boron Concentration 87 Pressure Relationships in Saturated Soil 57 Bicarbonate Concentration 87 Pressure Relationships in Unsaturated Soil 58 Salt Accumulation and Plant Response 89 THE SOIL WATER POTENTIAL 59 Salinity Control and Leaching Requirement 89 The Gravitational Potential 59 Effect of Irrigation on River Water Quality 93 The Matric Potential 59 Nature and Management of Saline and The Osmotic Potential 60 Sodic Soils 93 Measurement and Expression of Saline Soils 93 Water Potentials 60 Sodic Soils 93 Saline-Sodic Soils 94 SOIL WATER MOVEMENT 61 Water Movement in Saturated Soil 62 WASTEWATER DISPOSAL 94 Water Movement in Unsaturated Soil 63 Disposal of Septic Tank Effluent 94 Water Movement in Stratified Soil 63 Land Disposal of Municipal Wastewater 96 Water Vapor Movement 66 PRESCRIPTION ATHLETIC TURF 97 PLANT AND SOIL WATER RELATIONS 66 Available Water-Supplying Power of Soils 66 CHAPTER 7 Water Uptake from Soils by Roots 67 SOIL EROSION 100 Diurnal Pattern of Water Uptake 68 Pattern of Water Removal from Soil 69 Soil Water Potential Versus Plant Growth 69 Role of Water Uptake for Nutrient Uptake 71 WATER EROSION 100 Predicting Erosion Rates on Agricultural Land 100 SOIL WATER REGIME 71 R = The Rainfall Factor 101
DETAILED CONTENTS XI K = The Soil Erodibility Factor 102 Mycorrhiza 126 LS = The Slope Length and Slope Nitrogen Fixation 127 Gradient Factors 103 C = The Cropping-Management Factor 104 SOIL ORGANISMS AND ENVIRONMENTAL P = The Erosion Control Practice Factor 105 QUALITY 128 Pesticide Degradation 128 Application of the Soil-Loss Equation 106 The Soil Loss Tolerance Value 107 Oil and Natural Gas Decontamination 128 Water Erosion on Urban Lands 108 EARTH MOVING BY SOIL ANIMALS 130 Water Erosion Costs 109 Earthworm Activity 130 Ants and Termites 130 WIND EROSION 110 Types of Wind Erosion Rodents 131 110 Wind Erosion Equation 111 Factors Affecting Wind Erosion 111 Deep Plowing for Wind Erosion Control 113 CHAPTER 9 Wind Erosion Control on Organic Soils 113 SOIL ORGANIC MATTER 133 CHAPTER 8 THE ORGANIC MATTER IN ECOSYSTEMS 133 SOIL ECOLOGY 115 DECOMPOSITION AND ACCUMULATION 133 Decomposition of Plant Residues 134 Stable Soil Organic Matter THE ECOSYSTEM 115 Labile Soil Organic Matter 134 Producers 115 135 Consumers and Decomposers 116 Decomposition Rates 136 Properties of Stable Soil Organic Matter 136 MICROBIAL DECOMPOSERS 116 Protection of Organic Matter by Clay 137 General Features of Decomposers 116 Bacteria 117 ORGANIC SOILS 139 Fungi 117 Organic Soil Materials Defined 139 Actinomycetes 118 Formation of Organic Soils 139 Vertical Distribution of Decomposers in Properties and Use 140 the Soil 119 Archaeological Interest 140 SOIL ANIMALS 119 THE EQUILIBRIUM CONCEPT 141 Worms 120 A Case Example 141 Earthworms 120 Effects of Cultivation 142 Nematodes 121 Maintenance of Organic Matter in Arthropods 121 Cultivated Fields 143 Springtails 121 Effects of Green Manure 144 Mites 122 HORTICULTURAL USE OF ORGANIC MATTER 144 Millipedes and Centipedes 122 Horticultural Peats 145 White Grubs 122 Composts 145 Interdependence of Microbes and Animals i n Decomposition 123 NUTRIENT CYCLING 123 CHAPTER 10 Nutrient Cycling Processes 123 SOIL MINERALOGY 148 A Case Study of Nutrient Cycling 124 Effect of Crop Harvesting on Nutrient Cycling 124 SOIL MICROBE AND ORGANISM CHEMICAL AND MINERALOGICAL COMPOSITION OF I NTERACTIONS 125 THE EARTHS CRUST 148 The Rhizosphere 125 Chemical Composition of the Earths Crust 148 Disease 126 Mineralogical Composition of Rocks 149
DETAILED CONTENTSWEATHERING AND SOIL MINERALOGICAL MANAGEMENT OF SOIL pH 178COMPOSITION 149 Lime Requirement 179Weathering Processes 150 Lime Requirement of IntensivelySummary Statement 150 Weathered Soils 180Weathering Rate and Crystal Structure 151 Lime Requirement of Minimally and ModeratelyMineralogical Composition Versus Soil Age 153 Weathered Soils 180Summary Statement 155 The Liming Equation and Soil Buffering 181SOIL CLAY MINERALS 155 Some Considerations in Lime Use 182Mica and Vermiculite 156 Management of Calcareous Soils 182Smectites 158 Soil Acidulation 183Kaolinite 159 EFFECTS OF FLOODING ON CHEMICALAllophane and Halloysite 160 PROPERTIES 183Oxidic Clays 160 Dominant Oxidation and Reduction Reactions 183Summary Statement 161 Effect on Soil pH 184 CHAPTER 12ION EXCHANGE SYSTEMS OF SOIL CLAYS 161 161 PLANT-SOIL MACRONUTRIENTLayer Silicate SystemOxidic System 162Oxide-Coated Layer Silicate System 162 RELATIONS 186 DEFICIENCY SYMPTOMS 186CHAPTER 11SOIL CHEMISTRY 164 NITROGEN 186 The Soil Nitrogen Cycle 187 Dinitrogen Fixation 187CHEMICAL COMPOSITION OF SOILS 164 Symbiotic Legume Fixation 187 Nonlegume Symbiotic Fixation 192 Nonsymbiotic Nitrogen Fixation 192ION EXCHANGE 165 Summary Statement 192Nature of Cation Exchange 165Cation Exchange Capacity of Soils 166Cation Exchange Capacity Versus Soil pH 167 Mineralization 192Kinds and Amounts of Exchangeable Cations 168 Nitrification 193 I mmobilization 194 Carbon-Nitrogen Relationships 195Exchangeable Cations as a Source of PlantNutrients 169Anion Exchange 169 Denitrification 195SOIL pH 170 Human Intrusion in the Nitrogen Cycle 196Determination of Soil pH 170 Summary Statement on Nitrogen Cycle 197Sources of Alkalinity 170 Plant Nitrogen Relations 197 Carbonate Hydrolysis 170 PHOSPHORUS 197 Mineral Weathering 171 Soil Phosphorus Cycle 198Sources of Acidity 171 Effect of pH on Phosphorus Availability 199Development and Properties of Acid Soils 171 Changes in Soil Phosphorus Over Time 199 Role of Aluminum 172 Plant Uptake of Soil Phosphorus 200 Moderately Versus Intensively Plant Phosphorus Relations 201 Weathered Soils 173 Role of Strong Acids 174 POTASSIUM 202 Soil Potassium Cycle 202 Acid Rain Effects 174 Summary Statement 203Soil Buffer Capacity 174 Plant Uptake of Soil Potassium 204Summary Statement 176 Plant Potassium Relations 205SIGNIFICANCE OF SOIL pH 176 CALCIUM AND MAGNESIUM 205Nutrient Availability and pH 177 Plant Calcium and Magnesium Relations 206Effect of pH on Soil Organisms 178 Soil Magnesium and Grass Tetany 206Toxicities in Acid Soils 178pH Preferences of Plants 178 SULFUR 207
xiii DETAILED CONTENTS CHAPTER 13 APPLICATION AND USE OF FERTILIZERS 237 MICRONUTRIENTS AND Time of Application 238 TOXIC ELEMENTS 210 Methods of Fertilizer Placement 238 Salinity and Acidity Effects 240 IRON AND MANGANESE 210 ANIMAL MANURES 241 Plant "Strategies" for Iron Uptake 211 Manure Composition and Nutrient Value 241 Nitrogen Volatilization Loss from Manure 242 COPPER AND ZINC 212 Plant Copper and Zinc Relations 213 Manure as a Source of Energy 243 BORON 214 LAND APPLICATION OF SEWAGE SLUDGE 244 Sludge as a Nutrient Source 244 CHLORINE 214 Heavy Metal Contamination 245 MOLYBDENUM 214 FERTILIZER USE AND ENVIRONMENTAL Plant and Animal Molybdenum Relations 215 QUALITY 246 Phosphate Pollution 246 COBALT 216 Nitrate Pollution 246 SELENIUM 217 Nitrate Toxicity 247 POTENTIALLY TOXIC ELEMENTS SUSTAINABLE AGRICULTURE 247 FROM POLLUTION 217 CHAPTER 16 RADIOACTIVE ELEMENTS 218 SOIL GENESIS 250 CHAPTER 14 ROLE OF TIME IN SOIL GENESIS 250 FERTILIZERS 221 Case Study of Soil Genesis 250 Time and Soil Development Sequences 252 FERTILIZER TERMINOLOGY 221 ROLE OF PARENT MATERIAL IN SOIL GENESIS 253 Grade and Ratio 221 Consolidated Rock as a Source of General Nature of Fertilizer Laws 222 Parent Material 253 Types of Fertilizers 222 Soil Formation from Limestone Weathering 253 FERTILIZER MATERIALS 222 Sediments as a Source of Parent Material 254 Nitrogen Materials 222 Gulf and Atlantic Coastal Plains 255 Phosphorus Materials 224 Central Lowlands 256 Potassium Materials 226 Interior Plains 258 Micronutrient Materials 228 Basin and Range Region 258 Volcanic Ash Sediments 258 MIXED FERTILIZERS 228 Effect of Parent Material Properties on Granular Fertilizers 228 Soil Genesis 258 Bulk Blended Fertilizers 229 Stratified Parent Materials 259 Fluid Fertilizers 229 Parent Material of Organic Soils 260 NATURAL FERTILIZER MATERIALS 230 ROLE OF CLIMATE IN SOIL GENESIS 260 Precipitation Effects 260 CHAPTER 15 Temperature Effects 262 Climate Change and Soil Properties 263 SOIL FERTILITY EVALUATION AND FERTILIZER USE 232 ROLE OF ORGANISMS IN SOIL GENESIS 263 Trees Versus Grass and Organic Matter Content 263 SOIL FERTILITY EVALUATION 232 Vegetation Effects on Leaching and Eluviation 264 Plant Deficiency Symptoms 232 Role of Animals in Soil Genesis 265 Plant Tissue Tests 232 Soil Tests 233 ROLE OF TOPOGRAPHY IN SOIL GENESIS 265 Computerized Fertilizer Recommendations 235 Effect of Slope 265
xiv DETAILED CONTENTS Effects of Water Tables and Drainage 266 ANDISOLS 294 Topography, Parent Material, and Genesis and Properties 294 Time Interactions 267 Suborders 294 Uniqueness of Soils Developed in Alluvial ARIDISOLS 294 Parent Material 267 Genesis and Properties 295 HUMAN BEINGS AS A SOIL-FORMING FACTOR 269 Aridisol Suborders 295 Land Use on Aridisols 296 CHAPTER 17 ENTISOLS 297 SOIL TAXONOMY 271 Aquents 297 Fluvents 297 DIAGNOSTIC SURFACE HORIZONS 271 Psamments 298 Mollic Horizon 271 Orthents 298 Umbric and Ochric Horizons 272 HISTOSOLS 299 Histic Horizon 272 Histosol Suborders 299 Melanic Horizon 272 Land Use on Histosols 300 Anthropic and Plaggen Horizons 273 INCEPTISOLS 300 DIAGNOSTIC SUBSURFACE HORIZONS 273 Aquepts 300 Cambic Horizon 273 Ochrepts 301 Argillic and Natric Horizons 274 Umbrepts 301 Kandic Horizon 274 Spodic Horizon 275 MOLLISOLS 302 Albic Horizon 275 Aquolls 302 Oxic Horizon 275 Borolls 303 Calcic, Gypsic, and Salic Horizons 275 Ustolls and Udolls 303 Subordinate Distinctions of Horizons 276 Xerolls 304 SOIL MOISTURE REGIMES 276 OXISOLS 304 Aquic Moisture Regime 276 Oxisol Suborders 306 Udic and Perudic Moisture Regime 276 Land Use on Udox Soils 306 Ustic Moisture Regime 277 Land Use on Ustox Soils 307 Aridic Moisture Regime 277 Extremely Weathered Oxisols 307 Xeric Moisture Regime 278 Plinthite or Laterite 308 SOIL TEMPERATURE REGIMES 279 SPODOSOLS 308 Spodosol Suborders 308 CATEGORIES OF SOIL TAXONOMY 279 Spodosol Properties and Land Use 309 Soil Order 279 Suborder and Great Group 282 ULTISOLS 311 Subgroup, Family, and Series 283 Ultisol Suborders 311 Properties of Ultisols 311 AN EXAMPLE OF CLASSIFICATION: Land Use on Ultisols 312 THE ABAC SOILS 283 VERTISOLS 312 THE PEDON 284 Vertisol Suborders 313 Vertisol Genesis 313 CHAPTER 18 Vertisol Properties 314 SOIL GEOGRAPHY AND LAND USE 285 Land Use on Vertisols 315 CHAPTER 19 SOIL SURVEYS AND LAND-USE ALFISOLS 285 I NTERPRETATIONS 318 Aqualfs 285 Boralfs 286 Udalfs 286 Ustalfs 293 MAKING A SOIL SURVEY 318 Xeralfs 293 Making a Soil Map 318
DETAILED CONTENTS XVWriting the Soil Survey Report 320 FUTURE OUTLOOK 332Using the Soil Survey Report 321 Beyond Technology 333 The World Grain Trade 333SOIL SURVEY INTERPRETATIONS ANDLAND-USE PLANNING 322 Population Control and Politics 334Examples of Interpretative Land-Use Maps 322 APPENDIX ILand Capability Class Maps 323Computers and Soil Survey Interpretations 323 SOIL TEXTURE BY THE FIELD METHOD 337SOIL SURVEYS AND AGROTECHNOLOGYTRANSFER 324CHAPTER 20 APPENDIX IILAND AND THE WORLD TYPES AND CLASSES OFFOOD SUPPLY 326 SOIL STRUCTURE 339POPULATION AND FOOD TRENDS 326 APPENDIX IIIDevelopment of Agriculture 326The Industrial Revolution 326Recent Trends in Food Production 327 PREFIXES AND THEIR CONNOTATIONS FOR NAMES OF GREAT GROUPS IN THERecent Trends in Per Capita Cropland 328 U.S. SOIL CLASSIFICATION SYSTEMSummary Statement 329POTENTIALLY AVAILABLE LAND AND (SOIL TAXONOMY) 341SOIL RESOURCES 329 GLOSSARY 342Worlds Potential Arable Land 329 I NDEX 353Limitations of World Soil Resources 332Summary Statement 332
CHAPTER 1 SOIL AS A MEDIUM FOR PLANT GROWTHSOIL. Can you think of a substance that has had serve as channels for the movement of air andmore meaning for humanity? The close bond that water. Pore spaces are used as runways for smallancient civilizations had with the soil was ex- animals and are avenues for the extension andpressed by the writer of Genesis in these words: growth of roots. Roots anchored in soil suppportAnd the Lord God formed Man of dust from the plants and roots absorb water and nutrients. Forground. good plant growth, the root-soil environment should be free of inhibitory factors. The threeThere has been, and is, a reverence for the ground essential things that plants absorb from the soilor soil. Someone has said that "the fabric of hu- and use are: (1) water that is mainly evaporatedman life is woven on earthen looms; everywhere it from plant leaves, (2) nutrients for nutrition, andsmells of clay." Even today, most of the worlds (3) oxygen for root respiration.people are tillers of the soil and use simple toolsto produce their food and fiber. Thus, the conceptof soil as a medium of plant growth was born in Support for Plantsantiquity and remains as one of the most impor- One of the most obvious functions of soil is totant concepts of soil today (see Figure 1.1). provide support for plants. Roots anchored in soil enable growing plants to remain upright. Plants grown by hydroponics (in liquid nutrient culture)FACTORS OF PLANT GROWTH are commonly supported on a wire framework. Plants growing in water are supported by theThe soil can be viewed as a mixture of mineral buoyancy of the water. Some very sandy soils thatand organic particles of varying size and composi- are droughty and infertile provide plants with littletion in regard to plant growth. The particles oc- else than support. Such soils, however, producecupy about 50 percent of the soils volume. The high-yielding crops when fertilized and frequentlyremaining soil volume, about 50 percent, is pore irrigated. There are soils in which the impenetra-space, composed of pores of varying shapes and ble nature of the subsoil, or the presence of water-sizes. The pore spaces contain air and water and saturated soil close to the soil surface, cause shal- 1
z SOIL AS A MEDIUM FOR PLANT GROWTH FIGURE 1.1 Wheat harvest near the India-Nepal border. About one half of the worlds people are farmers who are closely tied to the land and make their living producing crops with simple tools.l ow rooting. Shallow-rooted trees are easily blownover by wind, resulting in windthrow. some plants. Plants deficient in an essential ele- ment tend to exhibit symptoms that are unique for that element, as shown in Figure 1.2.Essential Nutrient Elements More than 40 other elements have been found FIGURE 1.2Plants need certain essential nutrient elements to Manganese deficiency symptoms oncomplete their life cycle. No other element can kidney beans. The youngest, or upper leaves, havecompletely substitute for these elements. At least light-green or yellow-colored intervein areas and 16 elements are currently considered essential for dark-green veins.the growth of most vascular plants. Carbon, hy-drogen, and oxygen are combined in photosyn-thetic reactions and are obtained from air andwater. These three elements compose 90 percentor more of the dry matter of plants. The remaining13 elements are obtained largely from the soil.Nitrogen (N), phosphorus (P), potassium (K), cal-cium Ca), magnesium (Mg), and sulfur (S) arerequired in relatively large amounts and are re-ferred to as the macronutrients. Elements re-quired in considerably smaller amount are calledthe micronutrients. They include boron (B),chlorine (Cl), copper (Cu), iron (Fe), manganese(Mn), molybdenum (Mo), and zinc (Zn). Cobalt(Co) is a micronutrient that is needed by only
FACTORS OF PLANT GROWTH 3 i n plants. Some plants accumulate elements that Cations are positively charged ions such as Ca t are not essential but increase growth or quality. and K + and anions are negatively charged ions The absorption of sodium (Na) by celery is an such as NO3- (nitrate) and H 2 PO4 - (phosphate). example, and results in an improvement of flavor. The amount of cations absorbed by a plant is Sodium can also be a substitute for part of the about chemically equal to the amount of anions potassium requirement of some plants, if po- absorbed. Excess uptake of cations, however, re- tassium is in low supply. Silicon (Si) uptake may sults in excretion of H+ and excess uptake of i ncrease stem strength, disease resistance, and anions results in excretion of OH - or HCO 3 - to growth in grasses. maintain electrical neutrality in roots and soil. Most of the nutrients in soils exist in the miner- The essential elements that are commonly ab- als and organic matter. Minerals are inorganic sorbed from soils by roots, together with their substances occurring naturally in the earth. They chemical symbols and the uptake forms, are listed have a consistent and distinctive set of physical i n Table 1.1. properties and a chemical composition that can I n nature, plants accommodate themselves to be expressed by a formula. Quartz, a mineral the supply of available nutrients. Seldom or rarely composed of SiO2, is the principal constituent of is a soil capable of supplying enough of the essen- ordinary sand. Calcite (CaCO 3) i s the primary tial nutrients to produce high crop yields for any mineral in limestone and chalk and is abundant is reasonable period of time after natural or virgin many soils. Orthclase-feldspar (KAISi 3 O8 ) is a l ands are converted to cropland. Thus, the use ofvery common soil mineral, which contains po- animal manures and other amendments to in- tassium. Many other minerals exist in soils be- crease soil fertility (increase the amount of nutri-cause soils are derived from rocks or materials ent ions) are ancient soil management practices.containing a wide variety of minerals. Weathering of minerals brings about their decomposition andthe production of ions that are released into thesoil water. Since silicon is not an essential ele- Water Requirement of Plantsment, the weathering of quartz does not supply an A few hundred to a few thousand grams of wateressential nutrient, plants do not depend on these are required to produce 1 gram of dry plant mate-minerals for their oxygen. The weathering of cal- rial. Approximately one percent of this water be-cite supplies calcium, as Ca t+ , and the weather- comes an integral part of the plant. The remainderi ng of orthoclase releases potassium as K+. of the water is lost through transpiration, the loss The organic matter in soils consists of the re- of water by evaporation from leaves. Atmosphericcent remains of plants, microbes, and animals conditions, such as relative humidity and temper-and the resistant organic compounds resulting ature, play a major role in determining howfrom the rotting or decomposition processes. De- quickly water is transpired.composition of soil organic matter releases es- The growth of most economic crops will besential nutrient ions into the soil water where the curtailed when a shortage of water occurs, eveni ons are available for another cycle of plant though it may be temporary. Therefore, the soilsgrowth. ability to hold water over time against gravity is Available elements or nutrients are those nutri- i mportant unless rainfall or irrigation is adequate.ent ions or compounds that plants and microor- Conversely, when soils become water saturated,ganisms can absorb and utilize in their growth. the water excludes air from the pore spaces andNutrients are generally absorbed by roots as creates an oxygen deficiency. The need for thecations and anions from the water in soils, or the removal of excess water from soils is related to thesoil solution. The ions are electrically charged. need for oxygen.
4 SOIL AS A MEDIUM FOR PLANT GROWTH Chemical Symbols and Common Forms Oxygen Requirement of Plantsof the Essential Elements Absorbed by Plant RootsTABLE 1.1 Roots have openings that permit gas exchange.from Soils Oxygen from the atmosphere diffuses into the soil and is used by root cells for respiration. The car- bon dioxide produced by the respiration of roots, and microbes, diffuses through the soil pore space and exits into the atmosphere. Respiration releases energy that plant cells need for synthesis and translocation of the organic compounds needed for growth. Frequently, the concentration of nutrient ions in the soil solution is less than that i n roots cells. As a consequence, respiration energy is also used for the active accumulation of nutrient ions against a concentration gradient. Some plants, such as water lilies and rice, can grow in water-saturated soil because they have morphological structures that permit the diffusion of atmospheric oxygen down to the roots. Suc- cessful production of most plants in water culture requires aeration of the solution. Aerobic micro- organisms require molecular oxygen (O 2) and use oxygen from the soil atmosphere to decomposeFIGURE 1.3 The soil in which these tomato plants organic matter and convert unavailable nutrientswere growing was saturated with water. The stopper i n organic matter into ionic forms that plants canat the bottom of the left container was immediately reuse (nutrient cycling).removed, and excess water quickly drained away. Great differences exist between plants in theirThe soil in the right container remained water ability to tolerate low oxygen levels in soils. Sensi-saturated and the plant became severely wiltedwithin 24 hours because of an oxygen deficiency. FIGURE 1.4 Soil salinity (soluble salt) has seriously affected the growth of sugar beets in the foreground of this irrigated field.
PLANT ROOTS AND SOIL RELATIONS 5tive plants may be wilted and/or killed as a result etc.) in the shoot via photosynthesis and translo-of saturating the soil with water for a few hours, as cation of food downward for root growth, andshown in Figure 1.3. The wilting is believed to (2) the absorption of water and nutrients by rootsresult from a decrease in the permeability of the and the upward translocation of water and nutri-roots to water, which is a result of a disturbance of ents to the shoot for growth.metabolic processes due to an oxygen deficiency. After a root emerges from the seed, the root tip elongates by the division and elongation of cells i n the meristematic region of the root cap. AfterFreedom from Inhibitory Factors the root cap invades the soil, it continues to elon-Abundant plant growth requires a soil environ- gate and permeate the soil by the continued divi-ment that is free of inhibitory factors such as toxic sion and elongation of cells. The passage of thesubstances, disease organisms, impenetrable root tip through the soil leaves behind sections oflayers, extremes in temperature and acidity or root that mature and become "permanent" resi-basicity, or an excessive salt content, as shown in dents of the soil.Figure 1.4. As the plant continues to grow and roots elon- gate throughout the topsoil, root extension into the subsoil is likely to occur. The subsoil environ-PLANT ROOTS AND SOIL RELATIONS ment will be different in terms of the supply of water, nutrients, oxygen, and in other growth fac-Plants utilize the plant growth factors in the soil by tors. This causes roots at different locations in theway of the roots. The density and distribution of soil (topsoil versus subsoil) to perform differentroots affect the amount of nutrients and water that functions or the same functions to varying de-roots extract from soils. Perennials, such as oak grees. For example, most of the nitrogen willand alfalfa, do not reestablish a completely new probably be absorbed by roots from the topsoilroot system each year, which gives them a distinct because most of the organic matter is concen-advantage over annuals such as cotton or wheat. trated there, and nitrate-nitrogen becomes avail-Root growth is also influenced by the soil environ- able by the decomposition of organic matter. Byment; consequently, root distribution and density contrast, in soils with acid topsoils and alkalineare a function of both the kind of plant and the subsoils, deeply penetrating roots encounter anature of the root environment. great abundance of calcium in the subsoil. Under these conditions, roots in an alkaline subsoil may absorb more calcium than roots in an acid top-Development of Roots in Soils soil. The topsoil frequently becomes depleted ofA seed is a dormant plant. When placed in moist, water in dry periods, whereas an abundance ofwarm soil, the seed absorbs water by osmosis and water still exists in the subsoil. This results in aswells. Enzymes activate, and food reserves in the relatively greater dependence on the subsoil forendosperm move to the embryo to be used in water and nutrients. Subsequent rains that rewetgermination. As food reserves are exhausted, the topsoil cause a shift to greater dependence ongreen leaves develop and photosynthesis begins. the topsoil for water and nutrients. Thus, the man-The plant now is totally dependent on the sun for ner in which plants grow is complex and changesenergy and on the soil and atmosphere for nutri- continually throughout the growing season. Inents and water. In a sense, this is a critical period this regard, the plant may be defined as an inte-i n the life of a plant because the root system is grator of a complex and ever changing set ofsmall. Continued development of the plant re- environmental conditions.quires: (1) the production of food (carbohydrates, The root systems of some common agricultural
SOIL AS A MEDIUM FOR PLANT GROWTH FIGURE 1.6 Four stages for the development of the shoots and roots of corn (Zea maize). Stage one (left) shows dominant downward and diagonal root growth, stage two shows "filling" of the upper soil layer with roots, stage three shows rapid elongation of stem and deep root growth, and stage four (right) shows development of the ears (grain) and brace root growth.FIGURE 1.5 The tap root systems of two-week oldsoybean plants. Note the many fine roots branchingoff the tap roots and ramifying soil. (Scale on the systems and shoots during the growing seasonright is in inches.) revealed a synchronization between root and shoot growth. Four major stages of developmentcrops were sampled by using a metal frame to were found. Corn has a fibrous root system, andcollect a 10-centimeter-thick slab of soil from the early root growth is mainly by development ofwall of a pit. The soil slab was cut into small roots from the lower stem in a downward andblocks and the roots were separated from the soil diagonal direction away from the base of the plantusing a stream of running water. Soybean plants (stage one of Figure 1.6). The second stage of rootwere found to have a tap root that grows directly growth occurs when most of the leaves are de-downward after germination, as shown in Figure veloping and lateral roots appear and "fill" or1.5. The tap roots of the young soybean plants are space themselves uniformily in the upper 30 to 40several times longer than the tops or shoots. Lat- centimeters of soil. Stage three is characterized byeral roots develop along the tap roots and space rapid elongation of the stem and extension ofthemselves uniformily throughout the soil occu- roots to depths of 1 to 2 meters in the soil. Finally,pied by roots. At maturity, soybean taproots will during stage four, there is the production of theextend about 1 meter deep in permeable soils ear or the grain. Then, brace roots develop fromwith roots well distributed throughout the topsoil the lower nodes of the stem to provide anchorageand subsoil. Alfalfa plants also have tap roots that of the plant so that they are not blown over by thecommonly penetrate 2 to 3 meters deep; some wind. Brace roots branch profusely upon enteringhave been known to reach a depth of 7 meters. the soil and also function for water and nutrient Periodic sampling of corn (Zea maize) root uptake.
PLANT ROOTS AND SOIL RELATIONS 7 There is considerable uniformity in the lateral distribution of roots in the root zone of many crops (see Figure 1.7). This is explained on the basis of two factors. First, there is a random distri- bution of pore spaces that are large enough for root extension, because of soil cracks, channels formed by earthworms, or channels left from pre- vious root growth. Second, as roots elongate through soil, they remove water and nutrients, which makes soil adjacent to roots a less favor- able environment for future root growth. Then, roots grow preferentially in areas of the soil de- void of roots and where the supply of water and nutrients is more favorable for root growth. This results in a fairly uniform distribution of roots throughout the root zone unless there is some barrier to root extension or roots encounter an unfavorable environment. Most plant roots do not invade soil that is dry, nutrient deficient, extremely acid, or water satu- rated and lacking oxygen. The preferential devel- opment of yellow birch roots in loam soil, com- pared with sand soil, because of a more favorable FIGURE 1.8 Development of the root system of a yellow birch seedling in sand and loam soil. Both soils had adequate supplies of water and oxygen, but, the loam soil was much more fertile. (After Redmond, 1954.)FIGURE 1.7 Roots of mature oat plants grown inrows 7 inches (18 cm) apart. The roots made up 13percent of the total plant weight. Note the uniform,lateral, distribution of roots between 3 and 24 inches(8 and 60 cm). Scale along the left is in inches.Extensiveness of Roots in SoilRoots at plant maturity comprise about 10 percentof the entire mass of cereal plants, such as corn,wheat, and oats. The oat roots shown in Figure 1.7weighed 1,767 pounds per acre (1,979 kg/ha) andmade up 13 percent of the total plant weight. Fortrees, there is a relatively greater mass of roots ascompared with tops, commonly in the range of 15to 25 percent of the entire tree.
SOIL AS A MEDIUM FOR PLANT GROWTHcombination of nutrients and water, is shown in Roles of Root Interception, Mass Flow,Figure 1.8. and Diffusion Water and nutrients are absorbed at sites locatedExtent of Root and Soil Contact on or at the surface of roots. Elongating roots directly encounter or intercept water and nutrientA rye plant was grown in 1 cubic foot of soil for 4 ions, which appear at root surfaces in position formonths at the University of Iowa by Dittmer (this absorption. This is root interception and accountsstudy is listed amoung the references at the end of for about 1 percent or less of the nutrients ab-the chapter). The root system was carefully re- sorbed. The amount intercepted is in proportionmoved from the soil by using a stream of running to the very limited amount of direct root and soilwater, and the roots were counted and measured contact.for size and length. The plant was found to have Continued absorption of water adjacent to thehundreds of kilometers (or miles) of roots. Based root creates a lower water content in the soil nearon an assumed value for the surface area of the the root surface than in the soil a short distancesoil, it was calculated that 1 percent or less of the away. This difference in the water content be-soil surface was in direct contact with roots. tween the two points creates a water content gra-Through much of the soil in the root zone, the dient, which causes water to move slowly in thedistance between roots is approximately 1 cen- direction of the root. Any nutrient ions in the watertimeter. Thus, it is necessary for water and nutri- are carried along by flow of the water to rootent ions to move a short distance to root surfaces surfaces where the water and nutrients are both infor the effective absorption of the water and nutri- position for absorption. Such movement of nutri-ents. The limited mobility of water and most of the ents is called mass flow.nutrients in moist and well-aerated soil means The greater the concentration of a nutrient inthat only the soil that is invaded by roots can the soil solution, the greater is the quantity of thecontribute significantly to the growth of plants. nutrient moved to roots by mass flow. The range TABLE 1.2 Relation Between Concentration of Ions in the Soil Solution and Concentration within the Corn Plant Adapted from S. A. Barber, "A Diffusion and Mass Flow Concept of Soil Nutrient Availability, "Soil Sci., 93:39-49, 1962. Used by permission of the author and The Williams and Wilkins Co., Baltimore. a Dry weight basis.
SOIL FERTILITY AND SOIL PRODUCTIVITY 9of concentration for some nutrients in soil water is nutrient also depends on the amount needed. Cal-given in Table 1.2. The calcium concentration cium is rarely deficient for plant growth, partially(Table 1.2) ranges from 8 to 450 parts per million because plants needs are low. As a consequence, (ppm). For a concentration of only 8 ppm in soil these needs are usually amply satisfied by thewater, and 2,200 ppm of calcium in the plant, the movement of calcium to roots by mass flow. Theplant would have to absorb 275 (2,200/8) times same is generally true for magnesium and sulfur.more water than the plants dry weight to move the The concentration of nitrogen in the soil solutioncalcium needed to the roots via mass flow. Stated tends to be higher than that for calcium, but be-i n another way, if the transpiration ratio is 275 cause of the high plant demand for nitrogen, (grams of water absorbed divided by grams of about 20 percent of the nitrogen that plants ab-plant growth) and the concentration of calcium in sorb is moved to root surfaces by diffusion. Diffu-the soil solution is 8 ppm, enough calcium will be sion is the most important means by which phos-moved to root surfaces to supply the plant need. phorus and potassium are transported to root Because 8 ppm is a very low calcium concentra- surfaces, because of the combined effects of con-tion in soil solutions, and transpiration ratios are centration in the soil solution and plant demands. usually greater than 275, mass flow generally Mass flow can move a large amount of nutrients moves more calcium to root surfaces than plants rapidly, whereas diffusion moves a small amount need. In fact, calcium frequently accumulates of nutrients very slowly. Mass flow and diffusion along root surfaces because the amount moved to have a limited ability to move phosphorus and the roots is greater than the amount of calcium potassium to roots in order to satisfy the needs of that roots absorb. crops, and this limitation partly explains why a Other nutrients that tend to have a relatively l arge amount of phosphorus and potassium is l arge concentration in the soil solution, relative to added to soils in fertilizers. Conversely, the large the concentration in the plant, are nitrogen, mag- amounts of calcium and magnesium that are nesium, and sulfur (see Table 1.2). This means moved to root surfaces, relative to crop plant that mass flow moves large amount of these nutri- needs, account for the small amount of calcium ents to roots relative to plant needs. and magnesium that is added to soils in fertilizers. Generally, mass flow moves only a small amount of the phosphorus to plant roots. The phosphorus concentration in the soil solution is Summary Statement usually very low. For a soil solution concentration The available nutrients and available water are the of 0.03 ppm and 2,000 ppm plant concentration, nutrients and water that roots can absorb. The the transpiration ratio would need to be more than absorption of nutrients and water by roots is de- 60,000. This illustration and others that could be pendent on the surface area-density (cm 2 /cm 3 ) of drawn from the data in Table 1.2, indicate that roots. Mathematically: some other mechanism is needed to account for the movement of some nutrients to root surfaces. uptake = availability x surface area-density This mechanism or process is known as diffusion. Diffusion is the movement of nutrients in soil SOIL FERTILITY AND water that results from a concentration gradient. SOIL PRODUCTIVITY Diffusion of ions occurs whether or not water is Soil fertility is defined as the ability of a soil to moving. When an insufficient amount of nutrients is moved to the root surface via mass flow, diffu- sion plays an important role. Whether or not supply essential elements for plant growth with- plants will be supplied a sufficient amount of a out a toxic concentration of any element. Soil
10 SOIL AS A MEDIUM FOR PLANT GROWTHfertility refers to only one aspect of plant growth- and distribution of roots in soils, and (3) thethe adequacy, toxicity, and balance of plant nutri- movement of nutrients, water, and air to root sur-ents. An assessment of soil fertility can be made faces for absorption. Soils are productive in termswith a series of chemical tests. of their ability to produce plants. Soil productivity is the soils capacity to pro- The concept of soil as a medium for plantduce a certain yield of crops or other plants with growth views the soil as a material of fairly uni-optimum management. For example, the produc- form composition. This is entirely satisfactorytivity of a soil for cotton production is commonly when plants are grown in containers that containexpressed as kilos, or bales of cotton per acre, or a soil mix. Plants found in fields and forests,hectare, when using an optimum management however, are growing in soils that are not uniform.system. The optimum managment system spec- Differences in the properties between topsoil andi fies such factors as planting date, fertilization, subsoil layers affect water and nutrient absorp-irrigation schedule, tillage, cropping sequence, tion. It is natural for soils in fields and forests to beand pest control. Soil scientists determine soil composed of horizontal layers that have differentproductivity ratings of soils for various crops by properties, so it is also important that agricultur-measuring yields (including tree growth or timber i sts and foresters consider soils as natural bodies.production) over a period of time for those pro- This concept is also useful for persons involved induction uses that are currently relevant. Included the building of engineering structures, solving en-i n the measurement of soil productivity are the vironment problems such as nitrate pollution ofi nfluence of weather and the nature and aspect of groundwater, and using the soil for waste dis-slope, which greatly affects water runoff and ero- posal. The soil as a natural body is considered insion. Thus, soil productivity is an expression of all the next chapter.the factors, soil and nonsoil, that influence cropyields. For a soil to produce high yields, it must befertile for the crops grown. It does not follow, REFERENCEShowever, that a fertile soil will produce high Barber, S. A. 1962. "A Diffusion and Mass Flow Conceptyields. High yields or high soil productivity de- of Soil Nutrient Availability." Soil Sci. 93:39-49.pends on optimum managment systems. Many Dittmer, H. J. 1937. "A Quantitative Study of the Rootsfertile soils exist in arid regions but, within man- and Root Hairs of a Winter Rye Plant." Am. Jour. Bot.agement systems that do not include irrigation, 24:417-420. Foth, H. D. 1962. "Root and Top Growth of Corn."these soils are unproductive for corn or rice. Agron. Jour. 54:49-52. Foth, H. D., L. S. Robertson, and H. M. Brown. 1964. "Effect of Row Spacing Distance on Oat Perfor- mance." Agron. Jour. 56:70-73.SUMMARY Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. John Wiley, New York.The concept of soil as a medium for plant growth Redmond, D. R. 1954. "Variations in Development ofi s an ancient concept and dates back to at least Yellow Birch Roots in Two Soil Types." Forestrythe beginning of agriculture. The concept empha- Chronicle. 30:401-406.sizes the soils role in the growth of plants. Impor- Simonson, R. W. 1968. "Concept of Soil." Adv . in Agron.tant aspects of the soil as a medium for plant 20:1-47. Academic Press, New York.growth are: (1) the role of the soil in supplying Wadleigh, C. H. 1957. "Growth of Plants," in Soil, USDAplants with growth factors, (2) the development Yearbook of Agriculture. Washington, D.C.
CHAPTER 2 SOIL AS A NATURAL BODYOne day a colleague asked me why the alfalfa clay content than the two upper layers. The rootsplants on some research plots were growing so penetrated this layer with no difficulty, however.poorly. A pit was dug in the field and a vertical Below layer three, the alfalfa tap root encounteredsection of the soil was sampled by using a metal a layer (layer four) that was impenetrable (tooframe. The sample of soil that was collected was 5 compact), with the root growing above it in acentimeters thick, 15 centimeters wide, and 75 l ateral direction. From these observations it wascentimeters long. The soil was glued to a board concluded that the alfalfa grew poorly becauseand a vacuum cleaner was used to remove loose the soil material below a depth of 58 centimeters:soil debris and expose the natural soil layers and (1) created a barrier to deep root penetration,roots. Careful inspection revealed four soil layers which resulted in a less than normal supply ofas shown in Figure 2.1. water for plant growth during the summer, and The upper layer, 9 inches (22 cm) thick, is the (2) created a water-saturated zone above the thirdplow layer. It has a dark color and an organic l ayer that was deficient in oxygen during wet pe-matter content larger than any of the other layers. riods in the spring. The fact that the soil occurredLayer two, at the depth of 9 to 14 inches (22 to naturally in a field raises such questions as: What35 cm) differs from layer one by having a light-gray kinds of layers do soils have naturally? How do thecolor and a lower organic matter content. Both l ayers form? What are their properties? How dol ayers are porous and permeable for the move- these layers affect how soils are used? The an-ment of air and water and the elongation of roots. swers to these questions require an understand-I n layer three, at a depth of 14 to 23 inches (35 to i ng that landscapes consist of three-dimensional58 cm) many of the soil particles were arranged bodies composed of unique horizontal layers.i nto blocklike aggregrates. When moist soil from These naturally occurring bodies are soils. A rec-l ayer three was pressed between the fingers, more ognition of the kinds of soil layers and theirstickiness was observed than in layers one and properties is required in order to use soils effec-two, which meant that layer three had a greater tively for many different purposes. 11
12 SOIL AS A NATURAL BODY First is the formation of a parent material from which the soil evolves and, second, the evolution of soil layers, as shown in Figure 2.1. Approxi- mately 99 percent of the worlds soils develop in mineral parent material that was or is derived from the weathering of bedrock, and the rest de- velop in organic materials derived from plant growth and consisting of muck or peat. Bedrock Weathering and Formation of Parent Material Bedrock is not considered soil parent material because soil layers do not form in it. Rather, the unconsolidated debris produced from the weath- ering of bedrock is soil parent material. When bedrock occurs at or near the land surface, the weathering of bedrock and the formation of par- ent material may occur simultaneously with the evolution of soil layers. This is shown in Figure 2.2, where a single soil horizon, the topsoil layer, overlies the R layer, or bedrock. The topsoil layer i s about 12 inches (30 cm) thick and has evolved slowly at a rate controlled by the rate of rock weathering. The formation of a centimeter of soil i n hundreds of years is accurate for this example of soil formation. Rates of parent material formation from the di- rect weathering of bedrock are highly variable. A weakly cemented sandstone in a humid environ- ment might disintegrate at the rate of a centimeter i n 10 years and leave 1 centimeter of soil. Con- FIGURE 2.2 Rock weathering and the formation of the topsoil layer are occurring simultaneously. Scale is in feet.FIGURE 2.1 This alfalfa taproot grew verticallydownward through the upper three layers. At a depthof 23 inches (58 cm), the taproot encountered animpenetrable layer (layer 4) and grew in a lateraldirection above the layer.THE PARENT MATERIAL OF SOILSSoil formation, or the development of soils thatare natural bodies, includes two broad processes.
SOIL FORMATION 13versely, quartzite (metamorphosed sandstone) thick alluvial sediments occur in the valley. Verynearby might weather so slowly that any weath- thick glacial deposits occur on the tree-coveredered material might be removed by water or wind lateral moraine that is adjacent to the valley floorerosion. Soluble materials are removed during along the left side. An intermediate thickness ofli mestone weathering, leaving a residue of insolu- parent material occurs where trees are growingble materials. Estimates indicate that it takes below the bare mountaintops and above the thick 100,000 years to form a foot of residue from the alluvial and moraine sediments. Most of theweathering of limestone in a humid region. Where worlds soils have formed in sediments consistingsoils are underlain at shallow depths by bedrock, of material that was produced by the weathering l oss of the soil by erosion produces serious con- of bedrock at one place and was transported and sequences for the future management of the land. deposited at another location. In thick sediments or parent materials, the formation of soil layers isSediment Parent Materials not limited by the rate of rock weathering, and several soil layers may form simultaneously.Weathering and erosion are two companion andopposing processes. Much of the material lostfrom a soil by erosion is transported downslope SOIL FORMATIONand deposited onto existing soils or is added tosome sediment at a lower elevation in the land- Soil layers are approximately parallel to the landscape. This may include alluvial sediments along surface and several layers may evolve simulta-streams and rivers or marine sediments along neously over a period of time. The layers in a soilocean shorelines. Glaciation produced extensive are genetically related; however, the layers differsediments in the northern part of the northern from each other in their physical, chemical, andhemisphere. biological properties. In soil terminology, the lay- Four constrasting parent material-soil environ- ers are called horizons. Because soils as naturalments are shown in Figure 2.3. Bare rock is ex- bodies are characterized by genetically developedposed on the steep slopes near the mountaintops. horizons, soil formation consists of the evolutionHere, any weathered material is lost by erosion of soil horizons. A vertical exposure of a soil con-and no parent material or soil accumulates. Very sisting of the horizons is a soil profile. FIGURE 2.3 Four distinct soil- forming environments are depicted in this landscape in the Rocky Mountains, United States. On the highest and steepest slopes, rock is exposed because any weathered material is removed by erosion as fast as it forms. Thick alluvial sediments occur on the valley floor and on the forested lateral moraine adjacent to the valley floor along the left side. Glacial deposits of varying thickness overlying rock occur on the forested mountain slopes at intermediate elevations.
14 SOIL AS A NATURAL BODY Soil-Forming Processes animals feeding on the organic debris eventuallyHorizonation (the formation of soil horizons) re- die and thus contribute to the formation of hu-sults from the differential gains, losses, transfor- mus. Humus has a black or dark-brown color,mations, and translocations that occur over time which greatly affects the color of A horizons. Inwithin various parts of a vertical section of the areas in which there is abundant plant growth,parent material. Examples of the major kinds only a few decades are required for a surface layerof changes that occur to produce horizons are: to acquire a dark color, due to the humification(1) addition of organic matter from plant growth, and accumulation of organic matter, forming an Amainly to the topsoil; (2) transformation repre- horizon.sented by the weathering of rocks and minerals The uppermost horizons shown in Figures 2.1and the decomposition of organic matter; (3) loss and 2.2 are A horizons. The A horizon in Figure 2.1of soluble components by water moving down- was converted into a plow layer by frequent plow-ward through soil carrying out soluble salts; and, i ng and tillage. Such A horizons are called Ap(4) translocation represented by the movement of horizons, the p indicating plowing or other distur-suspended mineral and organic particles from the bance of the surface layer by cultivation, pastur-topsoil to the subsoil. i ng, or similar uses. For practical purposes, the topsoil in agricultural fields and gardens is synon-Formation of A and C Horizons ymous with Ap horizon. At this stage in soil evolution, it is likely that theMany events, such as the deposition of volcanic upper part of the underlying parent material hasash, formation of spoil banks during railroad con- been slightly altered. This slightly altered upperstruction, melting of glaciers and formation of part of the parent material is the C horizon. Theglacial sediments, or catastrophic flooding and soil at this stage of evolution has two horizons-formation of sediments have been dated quite the A horizon and the underlying C horizon. Suchaccurately. By studying soils of varying age, soil soils are AC soils; the evolution of an AC soil isscientists have reconstructed the kinds and the illustrated in Figure 2.4.sequence of changes that occurred to producesoils. Glacial sediments produced by continental and Formation of B Horizonsalpine glaciation are widespread in the northern The subsoil in an AC soil consists of the C horizonhemisphere, and the approximate dates of the and, perhaps, the upper part of the parent mate-formation of glacial parent materials are known. rial. Under favorable conditions, this subsoil layerAfter sediments have been produced near aretreating ice front, the temperature may becomefavorable for the invasion of plants. Their growth FIGURE 2.4 Sequential evolution of some soilresults in the addition of organic matter, espe- horizons in a sediment parent material.cially the addition of organic matter at or near thesoil surface. Animals, bacteria, and fungi feed onthe organic materials produced by the plants, re-sulting in the loss of much carbon as carbondioxide. During digestion or decomposition offresh organic matter, however, a residual organicfraction is produced that is resistant to furtheralteration and accumulates in the soil. The resis-tant organic matter is called humus and theprocess is humification. The microorganisms and
SOIL FORMATION 15 The Bt Horizon Soil parent materials frequently contain calcium carbonate (CaCO 3), or lime, and are alkaline. In the case of glacial parent materi- als, lime was incorporated into the ice when gla- ciers overrode limestone rocks. The subsequent melting of the ice left a sediment that contains li mestone particles. In humid regions, the lime dissolves in percolating water and is removed from the soil, a process called leaching. Leaching effects are progressive from the surface down- ward. The surface soil first becomes acid, and subsequently leaching produces an acid subsoil. An acid soil environment greatly stimulates mineral weathering or the dissolution of minerals with the formation of many ions. The reaction of orthoclase feldspar (KAISiO 3 ) with water and H+FIGURE 2.5 A soil scientist observing soil is as follows:properties near the boundary between the A and B 2 KAISiO3 + 9H 2 O + 2H +horizons in a soil with A, B, and C horizons. As roots (orthoclase)grow downward, or as water percolates downward,they encounter a different environment in the A, B,and C horizons. (Photograph USDA.) = H 4AI 2 Si 2 0 9 + 2K+ + 4H 4 Si04 (kaolinite) (silicic acid)eventually develops a distinctive color and some The weathering reaction illustrates three impor-other properties that distinguish it from the A hori- tant results of mineral weathering. First, clay parti-zon and underlying parent material, commonly at cles (fine-sized mineral particles) are formed-ina depth of about 60 to 75 centimeters. This altered the example, kaolinite. In effect, soils are "claysubsoil zone becomes a B horizon and develops factories. Second, ions are released into the soilas a layer sandwiched between the A and a new solution, including nutrient ions such as K + .deeper C horizon. At this point in soil evolution, Third, other compounds (silicic acid) of varyingi nsufficient time has elapsed for the B horizon to solubility are formed and are subject to leachinghave been significantly enriched with fine-sized and removal from the soil.(colloidal) particles, which have been translo- Clay formation results mainly from chemicalcated downward from the A horizon by percolat- weathering. Time estimates for the formation of 1i ng water. Such a weakly developed B horizon is percent clay inn rock parent material range fromgiven the symbol w (as in Bw), to indicate its 500 to 10,000 years. Some weathered rocks withweakly developed character. A Bw horizon can be small areas in which minerals are being con-distinguished from A and C horizons primarily by verted into clay are shown in Figure 2.6.color, arrangement of soil particles, and an inter- Many soil parent materials commonly containmediate content of organic matter. A soil with A, some clay. Some of this clay, together with clayB, and C horizons is shown in Figure 2.5. produced by weathering during soil formation, During the early phases of soil evolution, the tends to be slowly translocated downward fromsoil formation processes progressively transform the A horizon to the B horizon by percolatingparent material into soil, and the soil increases in water. When a significant increase in the claythickness. The evolution of a thin AC soil into a content of a Bw horizon occurs due to clay trans-thick ABwC soil is illustrated in Figure 2.4. l ocation, a Bw horizon becomes a Bt horizon.
16 SOIL AS A NATURAL BODY cles are believed to disperse when dry soil is wetted at the end of a dry season and the clay particles migrate downward in percolating water during the wet season. When the downward per- colating water encounters dry soil, water is with- drawn into the surrounding dry soil, resulting in the deposition of clay on the walls of pore spaces. Repeated cycles of wetting and drying build up layers of oriented clay particles, which are called clay skins. Many studies of clay illuviation have been made. The studies provide evidence that thou-FIGURE 2.6 sands of years are needed to produce a significant Weathering releases mineral grains in i ncrease in the content of clay in B horizons. Anrocks and results in the formation of very fine-sizedparticles of clay, in this case, kaolinite. example is the study of soils on the alluvial floodplain and adjacent alluvial fans in the Cen- tral Valley of California. Here, increasing eleva-Thin layers or films of clay can usually be ob- tion of land surfaces is associated with increasingserved along cracks and in pore spaces with a age. The soils studied varied in age from 1,000 to10-power hand lens. The process of accumulation more than 100,000 years.of soil material into a horizon by movement out of The results of the study are presented in Figuresome other horizon is illuviation. The t (as in Bt) 2.7. The Hanford soil developed on the floodplainrefers to an illuvial accumulation of clay. The Bt is 1,000 years old; it shows no obvious evidence ofhorizon may be encountered when digging holes illuviation of clay. The 10,000-year-old Greenfieldfor posts or trenching for laying underground soil has about 1.4 times more clay in the subsoilpipes. (Bt horizon) than in the A horizon. Snelling soils Alternating periods of wetting and drying seem are 100,000 years old and contain 2.5 times morenecessary for clay translocation. Some clay parti- clay in the Bt horizon than in the A horizon. The FIGURE 2.7 Clay distribution as a function of time in soils developed from granitic parent materials in the Central Valley of California. The Hanford soil, only 1,000 years old, does not have a Bt horizon. The other three soils have Bt horizons. The Bt horizon of the San Joaquin is a claypan that inhibits roofs and the downward percolation of water. (After Arkley, 1964.)
SOIL FORMATION 17San Joaquin soil is 140,000 years old, and has 3.4 The high content of sand results in soils withti mes more clay in the horizon of maximum clay l ow fertility and low water-retention capacityaccumulation as compared to the A horizon. (droughtiness). The three youngest soils (Hanford, Greenfield,and Snelling) are best suited for agriculture be-cause the subsoil horizons are permeable to wa- Formation of E Horizonster and air, and plant roots penetrate through the B The downward translocation of colloids from thehorizons and into the C horizons. Conversely, the A horizon may result in the concentration of sandi mpermeable subsoil horizon in San Joaquin soil and silt-sized particles (particles larger than claycauses shallow rooting. The root zone above the size) of quartz and other resistant minerals in thei mpermeable horizon becomes water saturated in upper part of many soils. In soils with thin Athe wet seasons. The soil is dry and droughty in horizons, a light-colored horizon may develop atthe dry season. the boundary of the A and B horizons (see Figure Water aquifiers underlie soils and varying thick- 2.4). This horizon, commonly grayish in color, isnesses of parent materials and rocks. Part of the the E horizon. The symbol E is derived from eluvi-precipitation in humid regions migrates com- ation, meaning, "washed-out." Both the A and Epletely through the soil and recharges underlying horizons are eluvial in a given soil. The mainaquifers. The development of water-impermeable feature of the A horizon, however, is the presenceclaypans over an extensive region results in less of organic matter and a dark color, whereas that ofwater recharge and greater water runoff. This has the E horizon is a light-gray color and having lowoccurred near Stuttgart, Arkansas, where wells organic matter content and a concentration of siltused for the irrigation of rice have run dry because and sand-sized particles of quartz and other resis-of the limited recharge of the aquifer. tant minerals. The development of E horizons occurs moreThe Bhs Horizon readily in forest soils than in grassland soils, be- Many sand parent materials cause there is usually more eluviation in forestcontain very little clay, and almost no clay forms soils, and the A horizon is typically much thinner.i n them via weathering. As a consequence, clay The development of E horizons occurs readily inilluviation is insignificant and Bt horizons do not soils with Bhs horizons, and the E horizons mayevolve. Humus, however, reacts with oxides of have a white color (see soil on book cover).aluminum and/or iron to form complexes in the A soil with A, E, Bt, and C horizons is shown inupper part of the soil. Where much water for Figure 2.8. At this building site, the suitability ofleaching (percolation) is present, as in humid the soil for the successful operation of a septicregions, these complexes are translocated down- effluent disposal system depends on the rate atward in percolating water to form illuvial accumu- which water can move through the least per-lations in the B horizon. The illuvial accumulation meable horizon, in this example the Bt hori-of humus and oxides of aluminum and/or iron in zon. Thus, the value of rural land for homethe B horizon produces Bhs horizons. The h indi- construction beyond the limits of municipalcates the presence of an illuvial accumulation of sewage systems depends on the nature of thehumus and the s indicates the presence of illuvial subsoil horizons and their ability to allow foroxides of aluminum and/or iron. The symbol s is the downward migration and disposal of sewagederived from sesquioxides (such as Fe 2 O 3 and effluent. Suitable sites for construction canA1 2 O3). Bhs horizons are common in very sandy be identified by making a percolation test ofsoils that are found in the forested areas of the those horizons through which effluent will beeastern United States from Maine to Florida. disposed.
18 SOIL AS A NATURAL BODY FIGURE 2.8 A soil with A, E, Bt, and C horizons that formed in 10,000 years under forest vegetation. The amount of clay in the Bt horizon and permeability of the Bt horizon to water determine the suitability of this site for home construction in a rural area where the septic tank effluent must be disposed by percolating through the soil.Formation of 0 Horizons The Soil-Forming FactorsVegetation produced in the shallow waters of Five soil-forming factors are generally recognized:lakes and ponds may accumulate as sediments of parent material, organisms, climate, topography,peat and muck because of a lack of oxygen in the and time. It has been shown that Bt and Bhswater for their decomposition. These sediments horizon development is related to the clay andare the parent material for organic soils. Organic sand content within the parent material and/orsoils have 0 horizons; the O refers to soil layers the amount of clay that is formed during soil evo-dominated by organic material. In some cases, l ution.extreme wetness and acidity at the surface of the Grass vegetation contributes to soils with thicksoil produce conditions unfavorable for decom- A horizons because of the profuse growth of fineposition of organic matter. The result is the forma- roots in the upper 30 to 40 centimeters of soil. Intion of O horizons on the top of mineral soil hori- forests, organic matter is added to soils mainly byzons. Although a very small proportion of the leaves and wood that fall onto the soil surface.worlds soils have O horizons, these soils are Small-animal activities contribute to some mixingwidely scattered throughout the world. of organic matter into and within the soil. As a result, organic matter in forest soils tends to be i ncorporated into only a thin layer of soil, result-SOILS AS NATURAL BODIES i ng in thin A horizons. The climate contributes to soil formationVarious factors contribute to making soils what through its temperature and precipitation com-they are. One of the most obvious is parent mate- ponents. If parent materials are permanently fro-rial. Soil formation, however, may result in many zen or dry, soils do not develop. Water is neededdifferent kinds of soils from a given parent mate- for plant growth, for weathering, leaching, andrial. Parent material and the other factors that are translocation of clay, and so on. A warm, humidresponsible for the development of soil are the climate promotes soil formation, whereas drysoil-forming factors. and/or cold climates inhibit it.
SOILS AS NATURAL BODIES 19 The topography refers to the general nature ofthe land surface. On slopes, the loss of water byrunoff and the removal of soil by erosion retardsoil formation. Areas that receive runoff watermay have greater plant growth and organic mattercontent, and more water may percolate throughthe soil. The extent to which these factors operate isa function of the amount of time that has beenavailable for their operation. Thus, soil may be de-fined as:unconsolidated material on the surface of theearth that has been subjected to and influenced FIGURE 2.10 Soil scientists studying soils asby the genetic and environmental factors of par- natural bodies in the field. Soils are exposed in theent material, climate, organisms, and topography, pit and soils data are displayed on charts forall acting over a period of time. observation and discussion.Soil Bodies as Parts of Landscapes slope (topography) account for the existence of different soil bodies in a given field, as shown inAt any given location on the landscape, there is a Figure 2.9. The dark-colored soil in the fore-particular soil with a unique set of properties, ground receives runoff water from the adjacenti ncluding kinds and nature of the horizons. Soil slopes. The light-colored soil on the slopes devel-properties may remain fairly constant from that oped where water runoff and erosion occurred.location in all directions for some distance. The Distinctly different management practices are re-area in which soil properties remain reasonably quired to use effectively the poorly drained soil inconstant is a soil body. Eventually, a significant the foreground and the eroded soil on the slope.change will occur in one or more of the soil- The boundary between the two different soilsforming factors and a different soil or soil body soils is easily seen. In many instances the bound-will be encountered. aries between soils require an inspection of the Locally, changes in parent material and/or soil, which is done by digging a pit or using a soil auger.FIGURE 2.9 A field or landscape containing black-colored soil in the lowest part of the landscape.This soil receives runoff water and eroded How Scientists Study Soils as Natural Bodiessediment from the light-colored soil on the slopingareas. A particular soil, or soil body, occupies a particu- l ar part of a landscape. To learn about such a soil, a pit is usually dug and the soil horizons are described and sampled. Each horizon is de- scribed in terms of its thickness, color, arrange- ment of particles, clay content, abundance of roots, presence or absence of lime, pH, and so on. Samples from each horizon are taken to the labo- ratory and are analyzed for their chemical, physi-
20 SOIL AS A NATURAL BODY FIGURE 2.11 Muck (organic) soil layer being replaced with sand to increase the stability of the roadbed.cal, and biological properties. These data are pre- moved and replaced with material that can with-sented in graphic form to show how various soil stand the pressures of vehicular traffic. The sceneproperties remain the same or change from one i n Figure 2.11 is of section of road that was builthorizon to another (shown for clay content in without removing a muck soil layer. The roadFigure 2.7). Figure 2.10 shows soil scientists became unstable and the muck layer was eventu-studying a soil in the field. Pertinent data are ally removed and replaced with sand.presented by a researcher, using charts, and theproperties and genesis of the soil are discussed by SUMMARYthe group participitants.I mportance of Concept of Soils as The original source of all mineral soil parent ma-Natural Bodies terial is rock weathering. Some soils have formed directly in the products of rock weathering at theirThe nature and properties of the horizons in a soil present location. In these instances, horizon for-determine the soils suitability for various uses. To mation may be limited by the rate of rock weather-use soils prudently, an inventory of the soils i ng, and soil formation may be very slow. Mostproperties is needed to serve as the basis for mak- soils, however, have formed in sediments result-i ng predictions of soil behavior in various situa- i ng from the erosion, movement, and depositiontions. Soil maps, which show the location of the of material by glaciers, water, wind, and gravity.soil bodies in an area, and written reports about Soils that have formed in organic sediments aresoil properties and predictions of soil behavior for organic soils.various uses began in the United States in 1896. By The major soil-forming processes include:the 1920s, soil maps were being used to plan the (1) humification and accumulation of organiclocation and construction of highways in Michi- matter, (2) rock and mineral weathering, (3)gan. Soil materials that are unstable must be re- l eaching of soluble materials, and (4) the eluvi-
REFERENCES 21ation and illuviation of colloidal particles. The REFERENCESoperation of the soil formation processes overti me produces soil horizons as a result of differen- Arkley, R. J. 1964. Soil Survey of the Eastern Stanislaus Area, California. U.S.D.A. and Cal. Agr. Exp. Sta.tial changes in one soil layer, as compared to Barshad, I. 1959. "Factors Affecting Clay Formation."another. The master soil horizons or layers include the Sciences Monograph 2. Pergamon, New York. Clays and Clay Minerals. E. Ingerson (ed.), EarthO, A, E, B, C, and R horizons. Jenny, H. 1941. Factors of Soil Formation. McGraw-Hill, Different kinds of soil occur as a result of the New York.i nteraction of the soil-forming factors: parent ma- Jenny, H. 1980. The Soil Resource. Springer-Verlag,terial, organisms, climate, topography, and time. New York. Landscapes are composed of three-dimen- Simonson, R. W. 1957. "What Soils Are." USDA Agricul-sional bodies that have naturally (genetically) de- tural Yearbook, Washington, D. C. Simmonson, R. W. 1959. "Outline of Generalized The-veloped horizons. These bodies are called soils. ory of Soil Genesis." Soil Sci. Soc. Am. Proc. 23:161- Prudent use of soils depends on a recognition 164.of soil properties and predictions of soil behavior Twenhofel, W, H. 1939. "The Cost of Soil in Rock andunder various conditions. Time." Am J. Sci. 237:771-780.
CHAPTER 3 SOIL PHYSICAL PROPERTIESPhysically, soils are composed of mineral and the size groups that are smaller than gravel. Theorganic particles of varying size. The particles are diameter and the number and surface area perarranged in a matrix that results in about 50 per- gram of the separates are given in Table 3.1.cent pore space, which is occupied by water and Sand is the 2.0 to 0.05 millimeter fraction and,air. This produces a three-phase system of solids, according to the United States Department of Agri-liquids, and gases. Essentially, all uses of soils are culture (USDA) system, the sand fraction is sub-greatly affected by certain physical properties. divided into very fine, fine, medium, coarse, andThe physical properties considered in this chapter very coarse sand separates. Silt is the 0.05 to 0.002i nclude: texture, structure, consistence, porosity, millimeter (2 microns) fraction. At the 0.05 milli-density, color, and temperature. meter particle size separation, between sand and silt, it is difficult to distinguish by feel the individ-SOIL TEXTURE ual particles. In general, if particles feel coarse or abrasive when rubbed between the fingers, theThe physical and chemical weathering of rocks particles are larger than silt size. Silt particles feeland minerals results in a wide range in size of smooth like powder. Neither sand nor silt is stickyparticles from stones, to gravel, to sand, to silt, when wet. Sand and silt differ from each other onand to very small clay particles. The particle-size the basis of size and may be composed of thedistribution determines the soils coarseness or same minerals. For example, if sand particles arefineness, or the soils texture. Specifically, texture smashed with a hammer and particles less thanis the relative proportions of sand, silt, and clay in 0.05 millimeters are formed, the sand has beena soil. converted into silt. The sand and silt separates of many soils areThe Soil Separates dominated by quartz. There is usually a signifi-Soil separates are the size groups of mineral parti- cant amount of weatherable minerals, such ascles less than 2 millimeters (mm) in diameter or feldspar and mica, that weather slowly and re- 22
SOIL TEXTURE 23 TABLE 3.1 Some Characteristics of Soil Separates Surface Number of Area in Diameter, Diameter, Particles I Gram, Separate mm mmb per Gram cm2 Very coarse sand 2.00-1.00 90 11 Coarse sand 1.00-0.50 2.00-0.20 720 23 Medium sand 0.50-0.25 5,700 45 Fine sand 0.25-0.10 0.20-0.02 46,000 91 Very fine sand 0.10-0.05 722,000 227 Silt 0.05-0.002 0.02-0.002 5,776,000 454 Clay Below 0.002 Below 0.002 90,260,853,000 8,000,000` a United States Department of Agriculture System. b I nternational Soil Science Society System. The surface area of platy-shaped montmorillonite clay particles determined by the glycol retention method by Sor and Kemper. (See Soil Science Society of America Proceedings, Vol. 23, p. 106, 1959.) The number of particles per gram and surface area of silt and the other separates are based on the assumption that particles are spheres and the largest particle size permissible for the separate.lease ions that supply plant needs and recombine statements apply to most clay in soils, some soilto form secondary minerals, such as clay. The clays have little tendency to show stickiness andgreater specific surface (surface area per gram) of expand when wetted. The clay fraction usuallysilt results in more rapid weathering of silt, com- has a net negative charge. The negative charge 2+pared with sand, and greater release of nutrient adsorbs nutrient cations, including Ca Mg2 +, +i ons. The result is a generally greater fertility in and K , and retains them in available form for usesoil having a high silt content than in soils high in by roots and microbes.sand content. Clay particles have an effective diameter lessthan 0.002 millimeters (less than 2 microns). Theytend to be plate-shaped, rather than sperical, and FIGURE 3.1 An electron micrograph of clayvery small in size with a large surface area per particles magnified 35,000 times. The platy shape, orgram, as shown in Table 3.1. Because the specific flatness, of the particles results in very high specificsurface of clay, is many times greater than that of surface. (Photograph courtesy Mineral Industries Experiment Station, Pennsylvania State University.)sand or silt, a gram of clay adsorbs much morewater than a gram of silt or sand, because wateradsorption is a function of surface area. The clayparticles shown in Figure 3.1 are magnified about30,000 times; their plate-shaped nature contrib-utes to very large specific surface. Films of waterbetween plate-shaped clay particles act as a lubri-cant to give clay its plasticity when wet. Con-versely, when soils high in clay are dried, there isan enormous area of contact between plate-shaped soil particles and great tendency for veryhard soil clods to form. Although the preceding
24 SOIL PHYSICAL PROPERTIESParticle Size Analysis is poured into a specially designed cylinder, andSieves can be used to separate and determine the distilled water is added to bring the contents up tocontent of the relatively large particles of the sand volume.and silt separates. Sieves, however, are unsatis- The soil particles settle in the water at a speedfactory for the separation of the clay particles from directly related to the square of their diameter andthe silt and sand. The hydrometer method is an inversely related to the viscosity of the water. Aempirical method that was devised for rapidly hand stirrer is used to suspend the soil particlesdetermining the content of sand, silt, and clay in a thoroughly and the time is immediately noted. Asoil. specially designed hydrometer is carefully in- In the hydrometer method a sample (usually 50 serted into the suspension and two hydrometergrams) of air-dry soil is mixed with a dispersing readings are made. The sand settles in about 40agent (such as a sodium pyrophosphate solution) seconds and a hydrometer reading taken at 40for about 12 hours to promote dispersion. Then, seconds determines the grams of silt and claythe soil-water suspension is placed in a metal cup remaining in suspension. Subtraction of the 40-with baffles on the inside, and stirred on a mixer second reading from the sample weight gives thefor several minutes to bring about separation of grams of sand. After about 8 hours, most of the siltthe sand, silt, and clay particles. The suspension has settled, and a hydrometer reading taken at 8 hours determines the grams of clay i n the sample (see Figure 3.2). The silt is calculated by differ-FIGURE 3.2 Inserting hydrometer for the 8-hour ence: add the percentage of sand to the percent-reading. The sand and silt have settled. The 8-hour age of clay and subtract from 100 percent.reading measures grams of clay in suspension and isused to calculate the percentage of clay. PROBLEM: Calculate the percentage of sand, clay, and silt when the 40-second and 8-hour hy- drometer readings are 30 and 12, respectively; assume a 50 gram soil sample is used: sample weight - 40-second reading x 100 = %sand sample weight 50 g - 30 g x 100 = 40% sand 509 8-hour reading x 100 = % clay sample weight 1 2 g x 100 = 24% clay 50 g 1 00% - (40% + 24%) = 36% silt After the hydrometer readings have been ob- tained, the soil-water mixture can be poured over a screen to recover the entire sand fraction. After it is dried, the sand can be sieved to obtain the various sand separates listed in Table 3.1.
SOIL TEXTURE 25 FIGURE 3.3 The textural triangle shows the limits of sand, silt, and clay contents of the various texture classes. Point A represents a soil that contains 65 percent sand, 15 percent clay, and 20 percent silt, and is a sandy loam.Soil Textural Classes arates-sand, silt, and clay. For sandy soils (sandOnce the percentages of sand, silt, and clay have and loamy sand), the properties and use of thebeen determined, the soil can be placed in one of soil are influenced mainly by the sand content of12 major textural classes. For a soil that contains the soil. For clays (sandy clay, clay, silty clay), the15 percent clay, 65 percent sand, and 20 percent properties and use of the soil are influencedsilt, the logical question is: What is the texture or mainly by the high clay content.textural class of the soil? Determining Texture by the The texture of a soil is expressed with the use of Field Methodclass names, as shown in Figure 3.3. The sum ofthe percentages of sand, silt, and clay at any pointin the triangle is 100. Point A represents a soil When soil scientists make a soil map, they use thecontaining 15 percent clay, 65 percent sand, and field method to determine the texture of the soil20 percent silt, resulting in a textural class name horizons to distinguish between different soils.of sandy loam. A soil containing equal amounts of When investigating a land-use problem, the abilitysand, silt, and clay is a clay loam. The area out- to estimate soil texture on location is useful inlined by the bold lines in the triangle defines a diagnosing the problem and in formulating a so-given class. For example, a loam soil contains 7 to lution.27 percent clay, 28 to 50 percent silt, and between A small quantity of soil is moistened with water22 and 52 percent sand. Soils in the loam class and kneaded to the consistency of putty to deter-are influenced almost equally by all three sep- mine how well the soil forms casts or ribbons
26 SOIL PHYSICAL PROPERTIES(plasticity). The kind of cast or ribbon formed isrelated to the clay content and is used to catego-rize soils as loams, clay loams, and clays. This isshown in Figure 3.4. If a soil is a loam, and feels very gritty or sandy,it is a sandy loam. Smooth-feeling loams are highi n silt content and are called silt loams. If thesample is intermediate, it is called a loam. Thesame applies to the clay loams and clays. Sandsare loose and incoherent and do not form rib-bons. More detailed specifications are given inAppendix 1.I nfluence of Coarse Fragments onClass NamesSome soils contain significant amounts of graveland stones and other coarse fragments that arel arger than the size of sand grains. An appropriateadjective is added to the class name in thesecases. For example, a sandy loam in which 20 to50 percent of the volume is made up of gravel is agravelly sandy loam. Cobbly and stony are usedfor fragments 7.5 to 25 centimeters and more than25 centimeters in diameter, respectively. Rocki- FIGURE 3.5 Foundation soil with high clay contentness expresses the amount of the land surface expanded and shrunk with wetting and drying,that is bedrock. cracking the wall. (Photograph courtesy USDA.)Texture and the Use of Soils closely related to soil texture. Many clay soilsPlasticity, water permeability, ease of tillage, expand and shrink with wetting and drying, caus-droughtiness, fertility, and productivity are all i ng cracks in walls and foundations of buildings, as shown in Figure 3.5. Conversely, many red- FIGURE 3.4 Determination of texture by the field colored tropical soils have clay particles com- method. Loam soil on left forms a good cast when posed mainly of kaolinite and oxides of iron and moist. Clay loam in center forms a ribbon that breaks alumimum. These particles have little capacity to somewhat easily. Clay on the right forms a long fexible ribbon. develop stickiness and to expand and contract on wetting and drying. Such soils can have a high clay content and show little evidence of stickiness when wet, or expansion and contraction with wet- ting and drying. Many useful plant growth and soil texture rela- tionships have been established for certain geo- graphic areas. Maximum productivity for red pine (Pinus resinosa) occurs on coarser-textured soil than for corn (Zea maize) i n Michigan. The great- est growth of red pine occurs on soils with sandy
SOIL STRUCTURE 27l oam texture where the integrated effects of the roots in subsoil Bt horizons. Water saturation ofnutrients, water, and aeration are the most desir- the soil above the Bt horizon may occur in wetable. Loam soils have the greatest productivity for seasons. Shallow rooting may result in a defi-corn. When soils are irrigated and highly fertil- ciency of water in dry seasons. A Bt horizon with aized, however, the highest yields of corn occur on high clay content and very low water permeabilitythe sand soils. The world record corn yield, set in is called a claypan. A claypan is a dense, compact 1977, occurred on a sandy soil that was fertilized l ayer in the subsoil, having a much higher clayand irrigated. content than the overlying material from which it The recommendations for reforestation in Ta- is separated by a sharply defined boundary (seeble 3.2 assume a uniform soil texture exists with Figure 3.6).i ncreasing soil depth, but this is often not true in Claypan soils are well suited for flooded ricethe field. Species that are more demanding of production. The claypan makes it possible towater and nutrients can be planted if the underly- maintain flooded soil conditions during the grow-i ng soil horizons have a finer texture. In some i ng season with a minimum amount of water.i nstances an underlying soil layer inhibits root Much of the rice produced in the United States ispenetration, thus creating a shallow soil with lim- produced on claypan soils located on nearly levelited water and nutrient supplies. land in Arkansas, Louisiana, and Texas. The other Young soils tend to have a similar texture in major kind of rice soil in the United States has veryeach horizon. As soils evolve and clay is translo- li mited water permeability because there is a claycated to the B horizon, a decrease in permeability texture in all horizons.of the subsoil occurs. Up to a certain point,however, an increase in the amount of clay in the SOIL STRUCTUREsubsoil is desirable, because the amount of waterand nutrients stored in that zone can also be in-creased. By slightly reducing the rate of water Texture is used in reference to the size of soilmovement through the soil, fewer nutrients will particles, whereas structure is used in reference tobe lost by leaching. The accumulation of clay in the arrangement of the soil particles. Sand, silt,ti me may become sufficient to restrict severely the and clay particles are typically arranged into sec-movement of air and water and the penetration of ondary particles called peds, or aggregates. The TABLE 3.2 Recommendations for Reforestation on Upland Soils in Wisconsin in Relation to Soil Textureª Percent Silt Plus Clay Species Recommended Less than 5 (Only for wind erosion control) 5-10 Jack pine, Red cedar 10-15 Red pine, Scotch pine, Jack pine 15-25 Increasing All pines 25-35 demand for White pine, European larch, Yellow birch, White water and elm, Red oak, Shagbark hickory, Black locust Over 35 nutrients White spruce, Norway spruce, White cedar, White ash, Basswood, Hard maple, White oak, Black walnut ª Wilde, S. A., "The Significance of Soil Texture in Forestry, and Its Determination by a Rapid Field Method," Journal of Forestry, 33:503-508, 1935. By permission Journal of Forestry.
28 SOIL PHYSICAL PROPERTIESshape and size of the peds determine the soils and the downward movement of water, however,structure. are not inhibited in the B horizon of most soils.I mportance of Structure Genesis and Types of StructureStructure modifies the influence of texture with Soil peds are classified on the basis of shape. Theregard to water and air relationships and the ease four basic structural types are spheroid, platelike,of root penetration. The macroscopic size of most blocklike, and prismlike. These shapes give risepeds results in the existence of interped spaces to granular, platy, blocky, and prismatic types ofthat are much larger than the spaces existing be- structure. Columar structure is prismatic-shapedtween adjacent sand, silt, and clay particles. Note peds with rounded caps.the relatively large cracks between the structural Soil structure generally develops from materialpeds in the claypan (Bt horizon) shown in Figure that is without structure. There are two structure-3.6. The peds are large and in the shape of blocks less conditions, the first of which is sands thator prisms, as a result of drying and shrinkage remain loose and incoherent. They are referred toupon exposure. When cracks are open, they are as single grained. Second, materials with a signif-avenues for water movement. When this soil wets, i cant clay content tend to be massive if they dohowever, the clay expands and the cracks be- not have a developed structure. Massive soil hastween the peds close, causing the claypan hori- no observable aggregation or no definite and or-zon to become impermeable. Root penetration derly arrangement of natural lines of weakness. FIGURE 3.6 A soil with a strongly developed Bt horizon; a claypan. On drying, cracks form between structural units. On wetting, the soil expands, the cracks close, and the claypan becomes i mpermeable to water and roots. (Photograph courtesy Soil Conservation Service, USDA.)
SOIL STRUCTURE 29Massive soil breaks up into random shaped clods 0. Structureless-no observable aggregration oror chunks. Structure formation is illustrated in no definite and orderly arrangement of natu-Figure 3.7. ral lines of weakness. Massive, if coherent, as Note in Figure 3.8 that each horizon of the soil i n clays. Single grained, if noncoherent, as inhas a different structure. This is caused by differ- sands.ent conditions for structure formation in each ho- 1. Weak-poorly formed indistinctive peds,rizon. The A horizon has the most abundant root barely observable in place.and small-animal activity and is subject to fre- 2. Moderate-well-formed distinctive peds,quent cycles of wetting and drying. The structure moderately durable and evident, but not dis-tends to be granular (see Figure 3.8). The Bt hori- tinct in undisturbed soil.zon has more clay, less biotic activity, and is 3. Strong-durable peds that are quite evidentunder constant pressure because of the weight of i n undisturbed soil, adhere weakly to one an-the overlying soil. This horizon is more likely to other, and become separated when the soil iscrack markedly on drying and to develop either a disturbed.blocky or prismatic structure. The development of The sequence followed in combining the threestructure in the E horizon is frequently weak and terms to form compound names is first the grade,the structure hard to observe. In some soils the E then the class, and finally the type. An example ofhorizon has a weakly developed platy structure a soil structural description is "strong fine(see Figure 3.8). granular."Grade and Class Managing Soil StructureComplete descriptions of soil structure include: Practical management of soil structure is gener-(1) the type, which notes the shape and arrange- ally restricted to the topsoil or plow layer in regardment of peds; (2) the class, which indicates ped to use of soils for plant growth. A stable structuresize; and (3) the grade, which indicates the dis- at the soil surface promotes more rapid infil-tinctiveness of the peds. The class categories de- tration of water during storms and results in re-pend on the type. A table of the types and classes duced water runoff and soil erosion and greaterof soil structure is given in Appendix 2. Terms for water storage within the soil. The permanence ofgrade of structure are as follows: peds depends on their ability to retain their shape FIGURE 3.7 Illustration of the formation of various types of soil structure from single- grained and massive soil conditions. Structure types are based on the shape of aggregates.
30 SOIL PHYSICAL PROPERTIES -- 30- - 60 -- -- 9O-- -- l2O--FIGURE 3.8 Structure of the horizons of a forest soilwith a Bt horizon. Scale is in centimeters.
SOIL CONSISTENCE 31after being subjected to the disruptive effects of stability. Practices that result in keeping a vegeta-raindrops and tillage. In addition, for peds to be tive cover also reduce raindrop impact and thuspermanent, the particles in the peds must hold reduce the disruption of peds.together when dry peds are wetted. SOIL CONSISTENCE When dry soil is suddenly wetted, water movesi nto the peds from all directions and compressesthe air in the pore spaces. Peds unable to with- Consistence is the resistance of the soil to defor-stand the pressure exerted by the entrapped air mation or rupture. It is determined by the cohe-are disrupted and fall apart. Unstable peds at the sive and adhesive properties of the entire soilsoil surface in cultivated fields are also broken mass. Whereas structure deals with the shape,apart by the force of falling raindrops. Disruption size, and distinctiveness of natural soil aggre-of peds produces smaller chunks and some indi- gates, consistence deals with the strength andvidual particles, which float over the soil surface nature of the forces between the sand, silt, andand seal off the surface to additional infiltrating clay particles. Consistence is important for tillagewater. On drying, the immediate surface soil may and traffic considerations. Dune sand exhibitsacquire a massive or structureless condition. minimal cohesive and adhesive properties, and Researchers at the University of Wisconsin because sand is easily deformed, vehicles canmeasured the development of peds in four soils in easily get stuck in it. Clay soils can become stickywhich they also determined the content of agents when wet, and thus make hoeing or plowing dif-known to bind particles together and give peds ficult.stability. These binding agents were microbial Soil Consistence Termsgum, organic carbon, iron oxide, and clay. Thedata in Table 3.3 show that microbial gum was Consistence is described for three moisture lev-the most important in promoting ped formation. els: wet, moist, and dry. A given soil may be stickyIron oxide, formed during weathering from min- when wet, firm when moist, and hard when dry. Aerals that contain iron, was most important in one partial list of terms used to describe consistencesoil and second most important when the four i ncludes:soils were grouped together. Soil managementpractices that result in frequent additions of or- 1. Wet soil-nonsticky, sticky, nonplastic,ganic matter to the soil in the form of plant resi- plasticdue, or manure, will tend to produce more micro- 2. Moist soil-loose, friable, firmbial gum and increase ped formation and 3. Dry soil-loose, soft, hard TABLE 3.3 Order of Importance of Soil Constituents in Formation of Peds over 0.5 mm in Diameter in the A Horizons of Four Wisconsin Soils Soil Order of Importance of Soil Constituents Parr silt loam Microbial gum > clay > iron oxide > organic carbon Almena silt loam Microbial gum > iron oxide Miami silt loam Microbial gum > iron oxide > organic carbon Kewaunee silt loam Iron oxide > clay > microbial gum All soils Microbial gum > iron oxide > organic carbon > clay Adapted from Chesters, G., O. J., Attoe, and O. N., Allen, "Soil Aggregation in Relation to Various Soil Constituents," Soil Science Society of America Proceedings Vol. 21, 1957, p. 276, by permission of the Soil Science Society of America.
32 SOIL PHYSICAL PROPERTIESPlastic soil is capable of being molded or de- Bulk samples of soil horizons are routinely col-formed continuously and permanently, by rela- l ected in the field and are used for chemical anal-tively moderate pressure, into various shapes yses and for some physical analyses, such aswhen wet. Friable soils readily break apart and are particle-size distribution. Soil cores are collectednot sticky when moist. to determine soil bulk density. The soil core Two additional consistence terms for special samples are collected to obtain a sample of soilsituations are cemented and indurated. Cementa- that has the undisturbed density and pore spacetion is caused by cementing agents such as cal- representative of field conditions. Coring ma-cium carbonate, silica, and oxides of iron and chines are used to remove soil cores with a diam-aluminum. Cemented horizons are not affected by eter of about 10 centimeters to a depth of a meterwater content and limit root penetration. When a or more. The technique for hand sampling of indi-cemented horizon is so hard that a sharp blow of a vidual soil horizons is shown in Figure 3.9. Carehammer is required to break the soil apart, the soilis considered to be indurated. A silica cemented horizon is called a duripan. FIGURE 3.9 Technique for obtaining soil cores thatLarge rippers pulled by tractors are used to break are used to determine bulk density. The light-coloredup duripans to increase rooting depth in soils. metal core fits into the core sampler just to the left.Sometimes a layer with an accumulation of car- This unit is then driven into the soil in pile-driverbonates (a Ck horizon, for example) accumulates fashion, using the handle and weight unit.so much calcium carbonate as to become ce-mented or indurated, and is transformed into apetrocalcic horizon. Petrocalcic horizons are alsocalled caliche. Cemented and indurated horizonstend to occur in soils of great age.DENSITY AND WEIGHT RELATIONSHIPSTwo terms are used to express soil density. Parti-cle density is the average density of the soil parti-cles, and bulk density is the density of the bulksoil in its natural state, including both the parti-cles and pore space.Particle Density and Bulk DensityA soil is composed of mineral and organic par-ticles of varying composition and density.Feldspar minerals (including orthoclase) are themost common minerals in rocks of the earthscrust and very common in soils. They have den-sities ranging from 2.56 g/cm 3 to 2.76 g/cm 3 .Quartz is also a common soil mineral; it has adensity of 2.65 g/cm 3 . Most mineral soils consistmainly of a wide variety of minerals and a smallamount of organic matter. The average particledensity for mineral soils is usually given as2.65 g/cm 3 .
DENSITY AND WEIGHT RELATIONSHIPS 33must be exercised in the collection of soil coresso that the natural structure is preserved. Anychange in structure, or compaction of soil, willalter the amount of pore space and, therefore, willalter the bulk density. The bulk density is the massper unit volume of oven dry soil, calculated asfollows: mass oven dry bulk density = volumePROBLEM: If the soil in the core of Figure 3.9weighs 600 grams over dry and the core has avolume of 400 cm3 , calculate the bulk density. bulk density = 400 0 3 = 1.5 g/cm 3 cm The bulk density of a soil is inversely related toporosity. Soils without structure, such as single-grained and massive soil, have a bulk density ofabout 1.6 to 1.7 g/cm 3 . Development of structure FIGURE 3.10 Bulk density and pore space of theresults in the formation of pore spaces between horizons of a soil with A, E, Bt, and C horizons.peds (interped spaces), resulting in an increase in (Data for Miami loam from Wascher, 1960.)the porosity and a decrease in the bulk density. Avalue of 1.3 g/cm3 is considered typical of loamsurface soils that have granular structure. As the tion exists, depending on the nature of the or-clay content of surface soils increases, there ganic matter and the moisture content at the timetends to be an increase in structural development of sampling to determine bulk density. Bulk den-and a decrease in the bulk density. As clay accu- sities for organic soils commonly range from 0.1mulates in B horizons, however, the clay fills ex- to 0.6 g/cm 3 .isting pore space, resulting in a decrease in pore Changes in soil porosity due to compaction arespace volume. As a result, the formation of Bt commonly evaluated in terms of changes in bulkhorizons is associated with an increase in bulk density.density. Weight of a Furrow-Slice of Soil The bulk densities of the various horizonsof a soil are given in Figure 3.10. In this soil theC horizon has the greatest bulk density,- Soil erosion losses are commonly expressed as1.7 g/cm3 . The C horizon is massive, or structurel- tons of soil per acre. An average annual soil lossess, and this is associated with high bulk density of 10 tons per acre is quite meaningless unless theand low porosity. Note that the Bt horizon is more weight of an inch of soil over 1 acre is known. Indense than the A horizon and that there is an this instance, an erosion rate of 10 tons annuallyi nverse relationship between bulk density and to- would result in the loss of a layer about 7 inchestal porosity (macropore space plus micropore thick, every 100 years.space). The acre-furrow-slice has customarily been Organic soils have very low bulk density considered to be a layer of soil about 7 inchescompared with mineral soils. Considerable varia- thick over 1 acre. An acre has 43,560 ft 2 . The
34 SOIL PHYSICAL PROPERTIESvolume of soil in an acre-furrow-slice, therefore, 100,000,000 cm 2 x 15 cm x 1.3 g/cm 3is 25,410 ft 3 to a depth of 7 inches. If the soil has a = 1,950,000,000 gbulk density of 1.3 g/cm 3 , the soil has a density The weight of a soil layer for a hectare that is 151.3 times that of water. Because a cubic foot of centimeters thick is, therefore, 1,950,000 kg/ha.water weighs 62.4 pounds, a cubic foot of the soilwould weigh 81.1 (62.4 x 1.3) pounds. Theweight of a 7-inch-thick acre-furrow-slice of a soil SOIL PORE SPACE AND POROSITYwith a bulk density of 1.3 g/cm3 (81 lb/ft3 ) is The fact that mineral soils have a particle densitycalculated as follows: and bulk density of about 2.65 and 1.3 g/cm 3 , 1 7/12 ft x 43 560 ft2 ] x [81.1 lb/ft 3 ] respectively, means that soils have about 50 per- = 2,061,259 lb cent total pore space or porosity. In simple terms, rocks without pore space are broken down byThis value-2,061,259-is usually rounded off as weathering to form mineral soils that have about2,000,000 pounds as the weight of an acre-furrow- 50 percent porosity. The pore spaces vary in size,slice of typical oven-dry loamy soil. and the size of pore spaces themselves can be as Soil samples to assess the fertility of a soil are i mportant as the total amount of pore space.usually taken to the depth of the plow layer. Theresults are commonly reported as pounds of po- Determination of Porositytassium, for example, per acre. Because it is usu-ally assumed that the plow layer of an acre weighs Oven dry soil cores, which have been used to2 million pounds, the test results are sometimes determine bulk density, can be placed in a pan ofreported as parts per 2 million (pp2m). For exam- water, allowed to saturate or satiate, and thenple, a soil test result for available potassium of 100 reweighed to obtain the data needed to calculatepounds per acre (acre-furrow-slice) is sometimes soil porosity. When the soil is satiated, the poregiven as 100 pp2m. In recent years there has been space is filled with water, except for a smalla tendency to plow deeper and, in many cases, the amount of entrapped air. The volume of water in afurrow slice is considered to be 9 or 10 inches satiated soil core approximates the amount ofthick and to weigh proportionally more. pore space and is used to calculate the soilsPROBLEM: Calculate the weight of a 10-inch- approximate porosity. For example, if the soil in athick acre-furrow-slice that has a bulk density of 400 cm2 core weighed 600 grams at oven dry, and1.2 g/cm 3 . 800 grams at water satiation, the satiated soil would contain 200 grams of water that occupies [ 10/12 ft x 43 560 ft] x [74.9 lb/ft 3 ] 200 cubic centimeters of space (1 g of water has a = 2,718,870 lb volume of 1 cm 3 ). Porosity is calculated asLime recommendations for increasing soil pH are follows:based on assumptions in regard to the weight of cm3 pore spacethe soil volume that must be limed. porosity = 3 X 100 cm soil volumeSoil Weight on a Hectare Basis 200 cm3 x 100 = 50%A hectare is an area 100 meters square and con- 400 cm3tains 10,000 square meters (m2) or 100,000,000cm 2 . A 15 centimeter thick layer of soil over a PROBLEM: A 500 cm 3 oven dry core has a bulkhectare that has a bulk density of 1.3 g/cm 3 would density of 1.1 g/cm 3 . The soil core is placed in aweigh pan of water and becomes water saturated. The
SOIL PORE SPACE AND POROSITY 35 It has been pointed out that sand surface soils have less porosity than clayey surface soils. Yet,oven dry soil and water at saturation weigh 825 our everyday experiences tell us that water movesgrams. Calculate the total soil porosity. much more rapidly through the sandy soil. The explanation of this apparent paradox lies in the pore size differences in the two soils. Sands con- tain mostly macropores that normally cannot re- tain water against gravity and are usually filled with air. As a consequence, macropores have also been called aeration pores. Since the porosity of sands is composed mainly of macropores, sands transmit water rapidly. Pores that are smallEffects of Texture and Structure enough to retain water against gravity will remainon Porosity water filled after soil wetting by rain or irrigation,Spheres in closest packing result in a porosity of and are called capillary or micropores.26 percent, and spheres in open packing have a Because sands have little micropore space theyporosity of 48 percent. Stated in another way, a are unable to retain much water. Fine-texturedball that just fits in a box occupies 52 percent of soils tend to contain mainly micropores and thusthe volume of the box, whereas 48 percent of the are able to retain a lot of water but have littlevolume is empty space. These facts are true re- ability to transmit water rapidly. An example of thegardless of size of the spheres or balls. Single- distribution of micropore and macropore spacegrained sands have a porosity of about 40 percent. i n the various horizons of a soil profile is given inThis suggests that the sand particles are not per- Figure 3.10. Note that the amounts of both totalfect spheres and the sand particles are not in a porosity and macropore space are inversely re-perfect close packing arrangement. The low po- l ated to the bulk density.rosity of single-grained sands is related to the Porosity and Soil Aerationabsence of structure (peds) and, therefore, anabsence of interped spaces. Fine-textured A horizons, or surface soils, have The atmosphere contains by volume nearly 79a wide range of particle sizes and shapes, and the percent nitrogen, 21 percent oxygen, and 0.03particles are usually arranged into peds. This percent carbon dioxide. Respiration of roots, andresults in pore spaces within and between other organisms, consumes oxygen and producespeds. These A horizons with well-developed gran- carbon dioxide. As a result, soil air commonlyular structure may have as much as 60 per- contains 10 to 100 times more carbon dioxide andcent porosity and bulk density values as low as slightly less oxygen than does the atmosphere1.0 g/cm3 . Fine-textured Bt horizons have a differ- (nitrogen remains about constant). Differences inent structural condition and tend to have less the pressures of the two gases are created beweenporosity and, consequently, a greater bulk density the soil and atmosphere. This causes carbonthan fine-textured A horizons. This is consistent dioxide to diffuse out of the soil and oxygen towith the filling of pore space by translocated clay diffuse into the soil. Normally, this diffusion isand the effects of the weight of the overlying soil, sufficiently rapid to prevent an oxygen deficiencywhich applies a pressure on the Bt horizon. The or a carbon dioxide toxicity for roots.pore spaces within the peds will generally be Although water movement through a uniformlysmaller than the pore spaces between the peds, porous medium is greatly dependent on pore size,resulting in a wide range in pore sizes. the movement and diffusion of gas are closely
36 SOIL PHYSICAL PROPERTIEScorrelated with total porosity. Gaseous diffusion susceptible to oxygen deficiency and may bei n soil, however, is also dependent on pore space killed when soils are water saturated for a fewcontinuity. When oxygen is diffusing through a hours (less than a day). Two yew plants that weremacropore and encounters a micropore that is planted along the side of a house at the same timefilled with water, the water-filled micropore acts are shown in Figure 3.11. The plant on the rightas a barrier to further gas movement. Diffusion of side died from oxygen stress caused by water-oxygen through the water barrier is essentially saturated soil as a result of flooding from thezero because oxygen diffusion through water is downspout water. The soil had a high clay con-about 10,000 times slower than through air. tent. The use of pipes to carry water away fromClayey soils are particularly susceptible to poor building foundations is also important in prevent-soil aeration when wet, because most of the pore i ng building foundation damage, where soils withspace consists of micropores that may be filled high clay content expand and contract because ofwith water. Sands tend to have good aeration or cycles of wetting and drying.gaseous diffusion because most of the porosity is When changes in soil aeration occur slowly,composed of macropores. In general, a desirable plants are able to make some adjustment. Allsoil for plant growth has a total porosity of 50 plant roots must have oxygen for respiration, andpercent, which is one half macropore porosity plants growing on flooded soil or in swamps haveand one half micropore porosity. Such a soil has a special mechanisms for obtaining oxygen.good balance between the retention of water forplant use and an oxygen supply for root res-piration. SOIL COLOR Oxygen deficiencies are created when soils be-come water saturated or satiated. The wilting of Color is about the most obvious and easily deter-tomato, due to water-saturated soil, is shown in mined soil property. Soil color is important be-Figure 1.3. Plants vary in their susceptibilty to cause it is an indirect measure of other importantoxygen deficiency. Tomatoes and peas are very characteristics such as water drainage, aeration, FIGURE 3.11 Both of these yews were planted at the same time along the side of a new house. The failure to i nstall a pipe on the right, to carry away the downspout water, caused water-saturated soil and killed the plant.
SOIL COLOR 37and the organic matter content. Thus, color is considerable decrease in organic matter contentused with other characteristics to make many im- with increasing soil depth. In some soils, finelyportant inferences regarding soil formation and divided manganese oxides contribute to blackland use. color. The major coloring agents of most subsoil hori- Determination of Soil Color zons are iron compounds in various states of oxi- dation and hydration. The rusting of iron is anSoil colors are determined by matching the color oxidation process that produces a rusty orof a soil sample with color chips in a Munsell reddish-colored iron oxide. The bright red colorsoil-color book. The book consists of pages, each of many tropical soils is due to dehydrated andhaving color chips arranged systematically ac- oxidized iron oxide, hematite (Fe2O3 ). Organiccording to their hue, value, and chroma, the three matter accumulation in the A horizons of thesevariables that combine to give colors. Hue refers soils results in a brownish-red or mahogany color.to the dominant wavelength, or color of the light. Hydrated and oxidized iron, goethite (FeOOH),Value, sometimes color brillance, refers to the has a yellow or yellowish-brown color, and re-quantity of light. It increases from dark to light duced and hydrated iron oxide has a gray color.colors. Chroma is the relative purity of the domi- Various combinations of iron oxides result innant wavelength of the light. The three properties brown and yellow-brown colors. Thus, the colorare always given in the order of hue, value, and of the iron oxides is related to aeration and hydra-chroma. In the notation, 10YR 6/4, 10YR is the tion conditions as controlled by the absence orhue, 6 is the value, and 4 is the chroma. This color presence of water. Soils on slopes that never satu-i s a light-yellowish brown. This color system en- rate with water have subsoils indicative of well-ables a person to communicate accurately the drained and aerated soil-subsoils with reddishcolor of a soil to anyone in the world. and brownish colors. Soils in depressions that collect water, and poorly drained locations inFactors Affecting Soil Color which soils are water saturated much of the time, will tend to have gray-colored B horizons. SoilsOrganic matter is a major coloring agent that af- i n intermediate situations will tend to havefects soil color, depending on its nature, amount, yellowish-colored B horizons. If a soil has a gray-and distribution in the soil profile. Raw peat is colored B horizon, the soil is likely to be waterusually brown; well-decomposed organic matter, saturated at least part of the time unless it issuch as humus, is black or nearly so. Many or- artificially drained (see Figure 3.12).ganic soils have a black color. The light and grayish colors of E horizons are In most mineral soils, the organic matter con- related to the illuviation of iron oxides and lowtent is usually greatest in the surface soil horizons organic matter content. Some soil horizons mayand the color becomes darker as the organic mat- have a white color because of soluble salt accu-ter content increases. Even so, many A horizons mulation at the soil surface or calcium carbonatedo not have a black color. Black-colored A hori- accumulation in subsoils. Horizons in young soilszons, however, are common in soils that devel- may be strongly influenced by the color of the soiloped under tall grass on the pampa of Argentina parent material.and the prairies of the United States. An interest-i ng color phenomenon occurs in the clayey soilsof the Texas Blacklands. The A horizon has a Significance of Soil Colorblack color; however, the black color may extend Many people have a tendency to equate black-to the depth of a meter even though there is a colored soils with fertile and productive soils.
38 SOIL PHYSICAL PROPERTIES color are a useful guide in selection of plant species. Alternating water saturation and drying of the subsoil, which may occur because of alternating wet and dry seasons, may produce an interme- diate color situation. During the wet season, iron i s hydrated and reduced, and during the dry sea- son the iron is dehydrated and oxidized. This causes a mixed pattern of soil colors called mot- tling. Mottled-colored B horizons are indicative of soils that are intermediate between frequent water saturation and soils with continuous well-drained conditions. SOIL TEMPERATURE Below freezing there is extremely limited biologi- cal activity. Water does not move through the soilFIGURE 3.12 Mineral soils that develop under the as a liquid and, unless there is frost heaving, timei nfluence of much soil wetness have a dark-colored stands still for the soil. A soil horizon as cold asA horizon and a gray-colored subsoil. The A horizon 5° C acts as a deterrent to the elongation of roots.is about 40 centimeters thick. The chemical processes and activities of microor- ganisms are temperature dependent. The al- ternate freezing and thawing of soils results in theBroad generalizations between soil color and soil alternate expansion and contraction of soils. Thisfertility are not always valid. Within a local region, affects rock weathering, structure formation, andi ncreases in organic matter content of surface the heaving of plant roots. Thus, temperature is ansoils may be related to increases in soil fertility, i mportant soil property.because organic matter is an important reservoirof nitrogen. Subsoil colors are very useful in predicting the Heat Balance of Soilslikelihood of subsoil saturation with water and The heat balance of a soil consists of the gainspoor aeration. Gray subsoil color indicates a fairly and losses of heat energy. Solar radiation receivedconstant water-saturated condition. Such soils are at the soil surface is partly reflected back into thepoor building sites because basements tend to be atmosphere and partly absorbed by the soil sur-wet and septic tank filter fields do not operate face. A dark-colored soil and a light-coloredproperly in water-saturated soil. Installation of a quartz sand may absorb about 80 and 30 percentdrainage system is necessary to use the soil as a of the incoming solar radiation, respectively. Ofbuilding site successfully. Subsoils that have the total solar radiation available for the earth,bright brown and red colors are indicative of good about 34 percent is reflected back into space, 19aeration and drainage. These sites are good loca- percent is absorbed by the atmosphere, and 47tions for buildings and for the production of tree percent is absorbed by the land.fruits. Many landscape plants have specific needs Heat is lost from the soil by: (1) evaporation ofand tolerances for water, and aeration and soil water, (2) radiation back into the atmosphere,
SOIL TEMPERATURE 39 (3) heating of the air above the soil, and (4) heat- within the normal rooting depth of trees, whereas i ng of the soil. For the most part the gains and soils on southern slopes do not. Trees tend to l osses balance each other. But during the daytime grow only on the south-facing slopes without per-or in the summer, the gains exceed the losses, manently frozen subsoils. Considerable variationwhereas the reverse is true for nights and winters. i n the microenvironment for plants can exist The amount of heat needed to increase the around a building. Southern exposures aretemperature of soil is strongly related to water warmer, drier, and have more light than northerncontent. It takes only 0.2 calories of heat energy to exposures. i ncrease the temperature of I gram of dry soil 1 ° C; Large bodies of water act as heat sinks andcompared with 1.0 calorie per gram per degree for buffer temperature changes nearby. In the springwater. This is important in the temperate regions and fall the temperature changes more slowly andwhere soils become very cold in winter and plant- gradually in the area adjacent to the Great Lakesi ng dates in the spring depend on a large rise in than at locations further inland. Near these lakessoil temperature. In general, sandy soils warm i n the spring, the temperatures of both air and soilmore quickly and allow earlier planting than do i ncrease slowly, which delays the blossoms onfine-textured soils, because sands retain less wa- fruit trees and thereby reduces the hazard of lateter and heat up faster. spring frosts. Killing frosts in the fall are delayed, resulting in an extension of the growing season. As a consequence, production of vine and treeLocation and Temperature fruits is concentrated in areas adjacent to theI n the northern hemisphere, soils located on Great Lakes. The effect of distance from Lake Eriesouthern slopes have a higher temperature than on the dates of last killing frost in spring and firstsoils on north-facing slopes. Soils on south-facing killing frost in the fall is shown in Figure 3.13.slopes are more perpendicular to the suns rays Control of Soil Temperatureand absorb more heat energy per unit area than dosoils on northern slopes. This is very obvious intundra regions where soils on north-facing slopes Two practical steps can be taken to change soilmay have permanently frozen subsoil layers temperature. Wet soils can be drained to removeFIGURE 3.13 Dates for last killing frost in spring and first killing frost in thefall at left and right, respectively, for Erie County, Pennsylvania. The longergrowing season along Lake Erie favors land use for fruits and vegetables.(Data from Taylor, 1960.)
40 SOIL PHYSICAL PROPERTIESwater, and mulches can be applied on the surfaceof the soil to alter the energy relationships. Whenused for agriculture, wet soils are drained tocreate an aerated root zone. The removal of wateralso causes the soil to warm more quickly in thespring. A light-colored organic matter mulch, such asstraw, will tend to lower soil temperature because(1) more solar radiation will be reflected and lessabsorbed, and (2) the water content of the soil willtend to be greater because more water will in-filtrate. The mulch will, however, tend to increasesoil temperature because the mulch tends to: FIGURE 3.14 Soil with permafrost having a layer of(1) reduce loss of heat by radiation, and (2) re- water-saturated soil overlying the permafrost. Even induce water evaporation from the soil surface, summer, with little precipitation, soils are wet. Frost heaving causes an irregular soil surface with roundedwhich requires energy. The net effect, however, is hummocks.to reduce soil temperature in the spring. In thenorthern corn belt, crop residues left on the soil The base of the active layer is the upper surfacesurface promote cooler soils in the spring and of the permafrost, or the permafrost table. Thetend to result in slightly lower corn yields. A black melting of frozen soil ice in summer results in wetplastic mulch increases soil temperature because soil conditions, even if there is little or no rain-it increases absorption of solar radiation, reduces fall, because the permafrost inhibits the down-heat loss by radiation, and reduces evaporation of ward movement of water from the soil. Alternatewater from the soil surface. Black mulches have freezing and thawing of wet soil above the perma-been used to increase soil temperatures in order frost produces cryoturbation (movement and mix-to produce vegetables and melons for an earliermarket. FIGURE 3.15 The utilidor system for servicing buildings in the Northwest Territories, Canada. UtilityPermafrost lines (water, sewage) in above-ground utilidor are protected from breakage resulting from soilWhen the mean annual soil temperature is below movement associated with freezing and thawing in0° C, the depth of freezing in winter may exceed soils with permafrost. The utilidor also heats thethe depth of thawing in summer. As a conse- water and sewage pipes to prevent freezing.quence, a layer of permanently frozen soil, calledpermafrost, may develop. Permafrost ranges frommaterial that is essentially all ice to frozen soil,which appears ordinary except that it is frozenand hard. The surface layer that thaws in summerand freezes in winter is the active l ayer. The activelayer is used by plants. Permafrost is widespreadi n tundra regions and where it occurs there is,generally, an absence of trees. Low-growingshrubs, herbs, and grasses grow on the soils withpermafrost if the temperature is above freezing inthe summer.
REFERENCES 41i ng of soil due to freezing and thawing). Soil wet- with favorable conditions for water retention andness from melting of ice and the irregular soil aeration, about one half of the porosity is macro-surface (hummocks) due to cryoturbation are pore space and one half is micropore space.shown in Figure 3.14. Excessive soil movement Soil color is used as an indicator of organicnecessitates the distribution of utilities to build- matter content, drainage, and aeration.i ngs above ground with utilidors, as shown in Soil temperature affects plant growth. Soil tem-Figure 3.15. perature is greatly affected by soil color, water When the vegetative cover is disturbed by traffic content, and the presence or absence of surfaceor building construction, more heat enters the soil materials, such as mulches. Permafrost occurs inand more ice melts. In some cases enough ice soils with average temperature below freezing.melts to cause soil subsidence. Buildings arebuilt on stilts to shade the soil in summer, topromote minimum heat transfer from buildings tosoil, and to allow maximum cooling and freezing REFERENCESof soil in winter. Chesters, G. 0., 0. J. Attoe, and 0. N. Allen. 1957. "Soil Aggregation in Relation to Various Soil Constitu- ents." Soil Sci. Soc. Am. Proc. 21:272-277. Day, P. R. 1953. "Experimental Confirmation of Hy-SUMMARY drometer Theory."Soil Sci. 75:181-186. Foth, H. D. 1983. "Permafrost in Soils." J Agron. Educ.Soil physical properties affect virtually every use 12:77-79.made of the soil. Soil Survey Staff. 1951. Soil Survey Manual. U.S.D.A. Texture relates to the amount of sand, silt, and Handbook 18. Washington, D.C.clay in the soil, and structure relates to the ar- Taylor, D. C. 1960. Soil Survey of Erie County, Pennsyl- vania. U.S.D.A. and Pennsylvania State Universityrangement of the sand, silt, and clay into peds. Tedrow, J. C. F. 1977. Soils of Polar Landscapes. Rut-Texture and structure greatly affect plant growth gers University Press, New Brunswick, N.J.by influencing water and air relationships. Soils Wascher, H. L. 1960. "Characteristics of Soils Associ-that expand and shrink with wetting and drying ated with Glacial Tills in Northeastern Illinois." Univ.affect the stability of building foundations. Illinois Agr. Epp. Sta. But. 665. About one half of the volume of mineral soils is Wilde, S. A. 1935. "The Significance of Texture in For-pore space. Such soils have a bulk density of estry and Its Determination by a Rapid Field Method."about 1.3 g/cm 3 and 50 percent porosity. In soils Jour. For. 33:503-508.
CHAPTER 4 TILLAGE AND TRAFFICThe beginning of agriculture marks the beginning monly accepted purposes of tillage: (1) to man-of soil tillage. Crude sticks were probably the first age crop residues, (2) to kill weeds, and (3) totillage tools used to establish crops by planting the alter soil structure, especially preparation of thevegetative parts of plants, including stem sec- soil for planting seeds or seedlings.tions, roots, and tubers. Paintings on the walls ofancient Egyptian tombs, dating back 5,000 years, Management of Crop Residuesdepict oxen yoked together pulling a plow madefrom a forked tree. By Roman times, tillage tools Crops are generally grown on land that containsand techniques had advanced to the point that the plant residues of a previous crop. The mold-thorough tillage was a recommended practice for board plow is widely used for burying these cropcrop production. Childe (1951) considered the residues in humid regions. Fields free of trashdiscovery and development of the plow to be one permit precise placement of seed and fertilizer atof the 19 most important discoveries or applica- planting and permit easy cultivation of the croptions of science in the development of civili- during the growing season (see Figure 4.1).zation. Now, paradoxically, the use of the plow for In subhumid and semiarid regions, by contrast,primary tillage is being challenged. the need for wind erosion control and conser- vation of moisture have led to the development ofEFFECTS OF TILLAGE ON SOILS AND tillage practices that successfully establish cropsPLANT GROWTH without plowing. The plant residues remaining on the soil surface provide some protection from wa-Tillage is the mechanical manipulation of soil for ter and wind erosion. Plant residues left on theany reason. In agriculture and forestry, tillage is l and over winter may trap snow and increase theusually restricted to the modification of soil con- water content of soils when the trapped snowditions for plant growth. There are three com- melts. 42
EFFECTS OF TILLAGE ON SOILS AND PLANT GROWTH 43 disintegration or breakdown remains unchanged. Consequently, tillage with cultivators, disks, and packers crushes some of the soil peds and tends to reduce soil porosity. Exposed cultivated land suffers from disruption of peds by raindrop im- pact in the absence of a vegetative cover. In addi- tion, tillage hastens organic matter decompo- sition. Therefore, the long-term effect of plow- i ng and cultivation is a more compacted soil as a result of the crushing of peds and subsequentFIGURE 4.1 Tillage (disking) in the spring to settling of soil.prepare the land for corn planting where the landwas plowed the previous fall and virtually all of the When forest or grassland soils are converted toprevious crop residues were buried. This type of use for crop production, there is a decline in soiltillage allows planting and cultivating without aggregation and soils become more compact be-interference from plant residues, but leaves the soil cause of the long-term effects of tillage. Compac-bare and susceptible to water runoff and soil erosion. tion results in a decrease in the total pore spaceSuch tillage is representative of conventional tillagesystems. and an increase in the bulk density. Pushing parti- cles together as a result of tillage results in a decrease in the average pore size. Some of theTillage and Weed Control macropores are reduced to micropores, and thus there is an increase in the volume of microporeWeeds compete with crop plants for nutrients, space and a decrease in the amount of macroporewater, and light. Weeds, however, can be con- space. The overall decrease in total porosity re-trolled with herbicides. If weeds are eliminated sults from a greater decrease in macropore spacewithout tillage, can cultivation of row crops be than the increase in micropore space. An increaseeliminated? Data from many experiments support i n micropore space, or water-filled space, be-the conclusion that the major benefit of culti- cause of the compaction of clayey soils, is detri-vating corn is weed control. Cultivation late in the mental because of reduced aeration and watergrowing season may be detrimental, because movement. By contrast, the compaction of a sandroots may be pruned and yields reduced. Culti- soil could be desirable because the soil couldvation to control weeds is important in sub- then retain more water and still be sufficientlyhumid regions to increase and conserve soil wa- aerated.ter when crops are not growing during a fallow A study was made of the effects of growingperiod. cotton for 90 years on Houston Black Clay soils in the U.S. Texas Blacklands. Soil samples from cul-Effects of Tillage on Structure tivated fields were compared with samples ob-and Porosity tained from an adjacent area that had remained uncultivated in grass. Long-time cultivationAll tillage operations change the structure of the caused a significant decrease in soil aggregationsoil. The lifting, twisting, and turning action of and total porosity and an increase in bulk density.a moldboard plow leaves the soil in a more ag- Changes in total porosity inversely paralleled thegregated condition. Cultivation of a field to kill changes in bulk density. These changes areweeds may also have the immediate effect of shown in Figure 4.2. The most striking and impor-l oosening the soil and increasing water infil- tant change was the decrease in macroporetration and aeration. The resistance of peds to space, which was reduced to about one half of
44 TILLAGE AND TRAFFIC FIGURE 4.2 Effects of 90 years of cultivation on pore space and bulk density (gm/ ³) CM of Houston Black Clay soils. Macropore and total pore space decreased and the micropore space increased. Decreases in total porosity resulted in increases in bulk density. (Data from Laws and Evans, 1949.)that of the uncultivated soil. Note that these grow well in experiments without tillage, the pro-changes occurred to a depth of at least 75 cen- duction of most crops will generally require someti meters. tillage. Sugar beet yields were lowest when the l and was not worked or tilled at all and when theSurface Soil Crusts l and was worked four or more times, between plowing and planting. The highest yields occurredDuring crop production, the surface of the soil is where the land was worked or tilled only once orexposed and left bare for varying periods of time twice between plowing and planting, as shown inwithout the protection of vegetation or crop resi- Table 4.1. The data in Table 4.1 show that exces-dues. Soil peds at the soil surface are broken apart sive tillage is detrimental. Some tillage is neces-by raindrops, and the primary particles (sand, silt, sary for the practical production of some crops;clay) are dispersed. The splashing raindrops and however, the total amount of tillage used is de-the presence of water cause the smallest particles clining. The result has been considerable pro-to move into the spaces between the larger parti-cles. A thin, dense-surface crust may form andreduce the water infiltration rate by a thousand- TABLE 4.1 Sugar Beet Yields as Affected by thefold or more. Thus, surface crust formation in- Number of Times a Field Was Worked Prior to Plantingcreases water runoff and erosion on sloping land.When the surface crust dries, the crust may be-come very hard and massive, inhibiting seedlingemergence. Crust hardness increases with in-creased clay content of the soil and the extent ofsoil drying.Minimum and Zero Tillage ConceptsIt is obvious that plants in natural areas growwithout tillage of the soil. Even though plants Adapted from Cook, et al., 1959.
EFFECTS OF TILLAGE ON SOILS AND PLANT GROWTH 45 I n no-till systems for row crop production, a narrow slot is made in the untilled soil so thatgress in the development of minimum tillage and seed can be planted where moisture is adequateno-till tillage systems. for germination. Weed control is by herbicides. Minimum tillage is the minimum soil manipu- The residues of the previous crop remain on thel ation necessary for crop production under the soil surface, which results in greatly reduced wa-existing soil and climatic conditions. Some of the ter runoff and soil erosion. There was 4,000minimum tillage equipment prepares the land for pounds per acre of wheat residue on the fieldplanting, plants the seeds, and applies fertilizer when the soybeans were planted in the fieldand herbicide in one trip over the field. Under shown in Figure 4.4. The coverage of the soilfavorable conditions, no further tillage will be re- surface with crop residues with various tillagequired during the growing season. Press wheels systems is shown in Table 4.2. Tillage systemsbehind the planter shoes pack soil only where the with reduced tillage and those where crop resi-seed is placed, leaving most of the soil surface in dues are left on the soil surface minimize watera state conducive to water infiltration. Experimen- runoff and soil erosion; this is called conservationtal evidence shows that minimum tillage reduceswater erosion on sloping land. It also makes weed control easier because planting immediately fol- tillage. lows plowing. Weed seeds left in loose soil are at From this discussion of tillage, it should be a disadvantage because the seeds have minimum obvious that tillage is not a requirement of plant contact with soil for absorbing water. Crop yields growth. Generally, gardens do not need to be are similar to those where more tillage is used, but plowed. The only tillage necessary is that needed the use of minimum tillage can reduce energy use for the establishment of the plants and the con- and overall production costs. Some of the sav- trol of weeds. Mulches in gardens, like crop resi- i ngs, conversely, may be used to pay for herbicide dues in fields, protect the soil from raindrop costs. i mpact and promote desirable soil structure. No-till is a procedure whereby a crop is planted A major benefit is a greater water- infitration directly into the soil with no preparatory tillage rate and a shorter amount of time needed to since the harvest of the previous crop. The plant- irrigate. i ng of wheat in the wheat stubble from the pre- vious crop without any prior tillage (such as plow- i ng) is shown in Figure 4.3. Tilth and Tillage Tilth is the physical condition of the soil as relatedFIGURE 4.3 to the ease of tillage, fitness of soil as a seedbed, No-till planting of wheat in the plantresidues of the previous years wheat crop. and desirability for seedling emergence and root(Photograph courtesy Soil Conservation Service, penetration. Tilth is related to structural condi-USDA.) tions, the presence or absence of impermeable l ayers, and the moisture and air content of the soil. The effect of tillage on tilth is strongly related to soil water content at the time of tillage. This is especially true when tillage of wet clay soils creates large chunks or clods that are difficult to break down into a good seedbed when the chunks dry. As shown earlier, cropping usually causes aggregate deterioration and reduced porosity. Farm managers need to exercise considerable j udgment concerning both the kind of tillage and the timing of tillage operations to minimize the
FIGURE 4.4 Soybeans planted no-till in the residues of a previous wheat crop. The previous crop left about 4,000 pounds of wheat residue per acre. (Photograph courtesy Soil Conservation Service, USDA.)detrimental effects and to maximize the beneficial soil compaction. These changes in porosity areeffects of tillage on soil tilth. i mportant enough to warrant their summarization as an aid to a further discussion of the effects of traffic on soils. Soil compaction results in: (1) aTRAFFIC AND SOIL COMPACTION decrease in total pore space, (2) a decrease in macropore space, and (3) an increase in micro-The changes in soil porosity, owing to compac- pore space.tion, are caused by the long-term effects ofcropping and tillage and by the pressure of tractortires, animal hooves, and shoes. As tractors andmachinery get larger, there is greater potential for Compaction Layers Compaction at the bottom of a plow furrow fre- quently occurs during plowing because of tractorTABLE 4.2 Effect of Tillage System on Amount of wheels running in a furrow that was made by theResidue Cover in Continuous Corn Immediately previous trip over the field. Subsequent tillage isAfter Planting usually too shallow to break up the compacted soil, and compaction gradually increases. This type of compaction produces a layer with high bulk density and reduced porosity at the bottom of the plow layer, and it is appropriately termed a plow pan, or pressure pan. Pressure pans are a problem on sandy soils that have insufficient clay content to cause enough swelling and shrinking, via wetting and drying, to naturally break up the compacted soil. Bulk densities as high as
TRAFFIC AND SOIL COMPACTION 471.9 g/cm ³ have been found in plow pans of sandy Potatoes are traditionally grown on sandy soilscoastal plain soils in the southeastern United that permit easy development of well-shapedStates. Cotton root extension was inhibited in tubers. Resistance to tuber enlargement in com-these soils when the bulk density exceeded pacted soils not only reduces tuber yield but in-1.6 g/cm3 . The effect of a compact soil zone on creases the amount of deformed tubers, whichthe growth of bean roots is shown in Figure 4.5. have greatly reduced market value. In studies where bulk density was used as a measure of soilEffects of Wheel Traffic on Soils compaction, potato tuber yield was negativelyand Crops correlated and the amount of deformed tubers was positively correlated with bulk density.As much as 75 percent of the entire area of an Effects of Recreational Trafficalfalfa field may be run over by machinery wheelsi n a single harvest operation. The potential forplant injury and soil compaction is great, because Maintenance of a plant cover in recreational areasthere are 10 to 12 harvests annually in areas where is necessary to preserve the natural beauty and toalfalfa is grown year-round with irrigation. Wheel prevent the undesirable consequences of watertraffic damages plant crowns, causing plants to runoff and soil erosion. Three campsites in thebecome weakened and more susceptible to dis- Montane zone of Rocky Mountain National Park inease infection. Root development is restricted. Colorado were studied to determine the effects ofAlfalfa stands (plant density) and yields have camping activities on soil properties. Soils inbeen reduced by wheel traffic. areas where tents were pitched, and where fire- places and tables were located, had a bulk densityFIGURE 4.5 Bean root growth in compacted soil at of 1.60 g/cm ³ compared with 1.03 g/cm³ i n areas4-inch depth on left and no soil compaction on the with little use. The results show that campingright. activities can compact soil as much as tractors and other heavy machinery. Snowmobile traffic on alfalfa fields significantly reduced forage growth at one out of four locations i n a Wisconsin study. It appears that injury to alfalfa plants and reduced yields are related to snow depth. There is less plant injury as snow depth increases. The large surface area of the snowmobile treads makes it likely that soil com- paction will be minimal or nonexistent. Moderate traffic on fields with a good snow cover appears to have little effect on soil properties or dormant plants buried in the snow. Human traffic compacts soil on golf courses and lawns. Aeration and water infiltration can be i ncreased by using coring aerators. Numerous small cores about 2 centimeters in diameter and 10 centimeters long are removed by a revolving drum and left lying on the ground, as shown in Figure 4.6. One of the most conspicuous recent changes in
48 TILLAGE AND TRAFFICsome of the arid landscapes of the southwestern disturb or break shallow tree roots that are grow-United States has been caused by use of all-terrain i ng just under the surface soil layer.vehicles. In the Mojave Desert one pass of a mo- The ease with which water can flow through thetorcycle increases bulk density of loamy sand soil was found to be 65 and 8 percent as great onsoils from 1.52 to 1.60 g/cm 3 , and 10 passes in- cutover areas and on logging roads, respectively,crease the bulk density to 1.68 g/cm 3 , according i n comparison with undisturbed areas in south-to one study. Parallel changes in porosity oc- western Washington. The reductions in perme-curred with the greatest reduction in the largest ability increase water runoff and soil erosion, re-pores. Water infiltration was decreased and water sulting in less water being available for the growthrunoff and soil erosion were greatly accelerated. of tree seedlings or any remaining trees. ReducedRecovery of natural vegetation and restoration of growth and survival of chlorotic Douglas-fir seed-soils occur very slowly in deserts. Estimates in- lings on tractor roads in western Oregon was con-dicate that about 100 years will be required to sidered to be caused by poor soil aeration and lowrestore bulk density, porosity, and infiltration nitrogen supply.capacity to their original values based on extrapo- Changes in bulk density and pore space of for-l ation of 51 years of data from an abandoned town est soils, because of logging, are similar toi n southern Nevada. changes produced by longtime cropping. After l ogging, and in the absence of further traffic,Effects of Logging Traffic on Soils and however, forest soils are naturally restored to theirTree Growth former condition. Extrapolation of 5 years data suggests that restoration of logging trails takes 8The use of tractors to skid logs has increased years and wheel ruts require 12 years in northernbecause of their maneuverability, speed, and Mississippi. In Oregon, however, soil compactioneconomy. About 20 to 30 percent of the forest land i n tractor skid trails was still readily observablemay be affected by logging. Tractor traffic can after 16 years. FIGURE 4.6 Soil aerator used to increase soil aeration on a college campus in California. The insert shows detail of removal of small cores that are left lying on the surface of the lawn.
TRAFFIC AND SOIL COMPACTION 49Controlled Traffic bulk density between a regular tillage systemThe pervasive detrimental effects of soil compac- using a moldboard plow and a conservation (re-tion have caused great interest in tillage research duced) tillage system with normal trafficking of(see Figure 4.7). The extensive coverage of the machinery wheels. When the tillage was carriedsoil surface by machinery wheels, producing soil out with the machinery wheels running on thecompaction, has resulted in the development of same path, no-wheel or controlled traffic, com-controlled traffic tillage systems. Controlled traffic paction caused by wheel traffic was eliminatedsystems have all tillage operations performed in and bulk densities were lower in both moldboardfixed paths so that recompaction of soil by traffic plow and conservation tillage than for wheel traf-(wheels) does not occur outside the selected fic. In the no-wheel traffic experiments, the con-paths. In fields with controlled traffic, only the servation tillage treatment eventually showedsmall amount of soil in the wheel tracks is com- l ower bulk densities (less compaction) thanpacted, and this compacted soil has almost no the moldboard plow treatment, as shown ineffect on crop yields. One additional benefit of Figure 4.8.controlled traffic is that the machines run on hard A controlled traffic experiment on sandy loamand firm paths that permit the planting of crops in soil in California showed that alfalfa yields withsoils with much greater water content than nor- normal traffic were 10 percent less as comparedmal. Tractors do not get stuck and crops can be to controlled traffic. The lower yields were alsoplanted at an earlier date. associated with more compact soils having An experiment was conducted in Minnesota to greater bulk density. An experimental controlledstudy the effects of machinery wheels on soil traffic machine for planting small grain is showncompaction in two tillage systems. The 9-year ex- i n Figure 4.9.periment showed there was no difference in soil In instances where repeated traffic is necessary, FIGURE 4.7 Tillage research facilities at the USDA National Soil Dynamics Laboratory in Auburn, Alabama. (Photograph courtesy of James H. Taylor.)
50 TILLAGE AND TRAFFIC percent of the rice is grown in Asia, mostly in small fields or paddies that have been leveled and bunded (enclosed with a ridge to retain water). The paddy fields are flooded and tilled before rice transplanting. A major reason for the tillage is the destruction of soil structure and formation of soil that is dense and impermeable to water. The prac- tice permits much land that is normally perme- able to water to become impermeable and suited for wetland rice production. In the United States, naturally water-impermeable soils are used for rice production and the soils are not puddled. A small amount of upland rice is grown, like wheat,FIGURE 4.8 Bulk density of the 0- to 15-centimeter on unflooded soils.soil layer as affected by tillage and wheel traffic over Effects of Floodingti me. (Reproduced from Soil Science Society AmericaJournal, Vol. 48, No. 1, January-February 1984, p.152-156 by permission of the Soil Science Society of Flooding of dry soil causes water to enter pedsAmerica.) and to compress the air in the pores, resulting in small explosions that break the peds apart. Theit is generally best to use the same tracks because anaerobic conditions (low oxygen supply) frommost of the compaction occurs with the first pass prolonged flooding cause the reduction and dis-over the soil. solution of iron and manganese compounds and the decomposition of organic structural-bindingFLOODING AND PUDDLING OF SOIL materials. Ped or aggregate stability is greatly re- duced and the aggregates are easily crushed. TheRice or paddy (wetland rice) is one of the worlds disruption of soil peds and the clogging of poresthree most important food crops: more than 90 with microbial wastes reduce soil permeability.FIGURE 4.9 I ncorporation of wheat seed at a depth Effects of Puddlingof 5 centimeters in a wet soil in a controlled trafficsystem where machinery wheels constantly run in the Puddling is the tillage of water-saturated soilsame tracks. (Photograph courtesy National Soil when water is standing on the field, as shown inDynamics Laboratory, USDA.) Figure 4.10. Peds that are already weakened by flooding are worked into a uniform mud, which becomes a two-phase system of solids and liq- uids. Human or animal foot traffic is effective in forming a compact layer, or pressure pan, about 5 to 10 centimeters thick at the base of the puddled l ayer. The development of a pressure pan allows for the conversion of soils with a wide range in texture and water permeability to become imper- meable enough for rice production. Puddling in- creases bulk density, eliminates large pores, and increases capillary porosity in the soil above the
FLOODING AND PUDDLING OF SOIL 51 FIGURE 4.10 Puddling soil to prepare land for paddy (rice) transplanting.pressure pan. Soil stratification may occur in soils Oxygen Relationships in Flooded Soilswhen sand settles before the silt and clay. This The water standing on a flooded soil has an oxy-results in the formation of a thin surface layer gen content that tends toward equilibrium withenriched with silt and clay, also having low per- the oxygen in the atmosphere. This oxygenatedmeability. These changes are desirable for rice water layer supplies oxygen via diffusion to a thinproduction because soil permeability is reduced upper layer of soil about a centimeter thick, asby a factor of about 1,000 and the amount of water shown in Figure 4.11. This thin upper layer retainsneeded to keep the field flooded is greatly re- the brownish or reddish color of oxidized soil.duced. Much of the rice is grown without a de- The soil below the thin surface layer and abovependable source of irrigation water, and rainfall the pressure pan is oxygen deficient, and reduc-can be erratic. Low soil permeability is, therefore, i ng conditions exist. Colors indicative of reducedof great importance in maintaining standing water iron and manganese occur, that is, gray and blackon the paddy during the growing season. Standing colors. The soil in the immediate vicinity of ricewater promotes rice growth (because of no water roots is oxidized because of the transport of oxy-stress and an increased supply of nutrients), and gen into the root zone from the atmosphere. Thisthe nearly zero level of soil oxygen inhibits growth produces a thin layer of yellowish soil around theof many weeds. roots where reduced (ferrous) iron has been oxi- Paddies are drained and allowed to dry before dized to ferric iron and precipitated at the rootharvest. Where winters are dry and no irrigation surfaces.water is available, a dryland crop is frequently The soil below the pressure pan can be eitherplanted. Drying of puddled soil promotes ped for- reduced or oxidized. If the soil occurs in a depres-mation; however, large clods often form that are sion and is naturally poorly drained, a gray sub-difficult to work into a good seedbed. Pressure soil that is indicative of reduced soil is likely.pans are retained from year to year and create a Many rice paddies are formed high on the land-shallow root zone that severely limits the supply scape, and they naturally have well-aerated sub-of water and nutrients for dry season crops. soils below pressure pans that may retain their
52 TILLAGE AND TRAFFIC All soil compaction results in higher bulk den- sity and lower total porosity. The volume of mac- ropores is greatly reduced with an accompanying i ncrease in micropores. Soil layers with a bulk ³ density greater than about 1.6 to 1.8 g/cm are barriers for root penetration. Pressure pans, or plow soles, tend to develop at the bottom of plow furrows, especially in sandy soils with minimal capacity for expansion and contraction due to wetting and drying. Minimum and no-till tillage systems have been developed to minimize the effects of tillage and traffic on soil compaction. Only the minimum amount of tillage necessary for the crop and cli- matic conditions is used in the first method. In no-till, crops are planted directly in the residues of the previous crop with no prior tillage. For row crops, a slit is made in the soil in which the seed is planted. Minimum and no-till tillage leave more residues on the soil surface than conventional tillage, resulting in reduced water runoff and soil erosion, thus they are considered conservation tillage. I n Asia, many soils are flooded and puddled to create a soil with minimal water permeability for flooded rice production. A pressure pan is created below a puddled soil layer (root zone).FIGURE 4.11 Oxidized and reduced zones inflooded paddy soil. REFERENCESbright oxidized colors after paddies are estab- Childe, V. E. 1951. Man Makes Himself. Mentor, Newlished. York. Cook, R. L., J. F. Davis, and M. G. Frakes. 1959. "An ers in Michigan." Mi. Agr. Exp. Sta. Quart. Bul. Analysis of Production Practices of Sugar Beet Farm-SUMMARY 42:401-420. Dickerson, B. P. 1976. "Soil Changes Resulting fromTillage is the mechanical manipulation of soil to Tree-Length Skidding." Soil Sci. Soc. Am. Proc.modify soil conditions for plant growth. Tillage is 40:965-966.beneficial for preparing land for planting, control- Dotzenko, A. D., N. T. Papamichos, and D. S. Romine.ling weeds, and managing crop residues. Tillage 1967. "Effect of Recreational Use on Soil and Mois-involves mechanical manipulation of soil and ve- ture Conditions in Rocky Mountain National Park."hicular or animal traffic, which results in soil J. Soil and Water Con. 22:196-197.compaction. Froehlich, H. A. 1979. "Soil Compaction from Logging
REFERENCES 53 Equipment: Effects on Growth of Young Ponderosa Phillips, R. E., R. L. Blevins, G. W. Thomas, W. R. Frye, Pine." J. Soil and Water Con. 34:276-278. and S. H. Phillips. 1980. "No-Tillage Agriculture."Griffith, D. R., J. V. Mannering, and W. C. Moldenhauer. Science. 208:1108-1113. 1977. "Conservation Tillage in the Eastern Corn Belt." Steinbrenner, E. C. and S. P. Gessel. 1955. "The Effect of J. Soil and Water Con. 32:20-28. Tractor Logging on Physical Properties of Some For-Grimes, D. W. and J. C. Bishop. 1971. "The Influence est Soils in Southwestern Washington." Soil Sci. Soc. of Some Soil Physical Properties on Potato Yields A. Proc. 19:372-376. and Grade Distribution." Am. Potato 1 48:414- Taylor, J. H. 1986. "Controlled Traffic: A Soil Compac- 422. tion Management Concept." SAE Tech. Paper SeriesIverson, R. M., B. S. Hinckley, and R. M. Webb. 1981. 860731. Soc. Auto. Eng. Inc., Warrendale, Pa. "Physical Effects of Vehicular Disturbances on Arid Voorhees, W. B. and M. J. Lindstrom. 1984. "Long-Term Landscapes." Science. 212:915-916. Effects of Tillage Method on Soil Tilth Independent ofLaws, W. D. and D. D. Evans. 1949. "The Effects of Wheel Traffic Compaction." Soil Sci. Soc. Am. J. Long-Time Cutltivation on Some Physical and Chemi- 48:152-156. cal Properties of Two Rendzina Soils." Soil Sci. Soc. Walejko, R. N., J. W. Pendleton, W. H. Paulson, R. E. Am. Proc. 14:15-19. Rand, G. H. Tenpas, and D. A. Schlough. 1973.Meek, B. D., E. A. Rechel, L. M. Carter, and W. R. DeTar. "Effect of Snowmobile Traffic on Alfalfa." J. Soil and 1988. "Soil Compaction and its Effect on Alfalfa in Water Con. 28:272-273. Zone Production Systems." Soil Sci. Soc. Am. J. Youngberg, C. T. 1959. "The Influence of Soil Condi- 52:232-236. tions Following Tractor Logging on the Growth ofMoormann, F. R. and N. van Breemen. 1978. Rice: Soil, Planted Douglas-Fir Seedlings." Soil Sci. Soc. Am. Water and Land. Int. Rice Res. Inst., Manila. Proc. 23:76-78.
CHAPTER 5 SOIL WATERWater is the most common substance on the SOIL WATER ENERGY CONTINUUMearth; it is necessary for all life. The supply offresh water on a sustained basis is equal to As water cascades over a dam, the potentialthe annual precipitation, which averages 66 cen- energy (ability of the water to do work) decreases.ti meters for the worlds land surface. The soil, If water that has gone over a dam is returned to thelocated at the atmosphere-lithosphere interface, reservoir, work will be required to lift the waterplays an important role in determining the back up into the reservoir, and the energy contentamount of precipitation that runs off the land and of the water will be restored. Therefore, in a ca-the amount that enters the soil for storage and scading waterfall, the water at the top has thefuture use. Approximately 70 percent of the pre- greatest energy and water at the bottom has thecipitation in the United States is evaporated from l owest energy. As water cascades down a falls, theplants and soils and returned to the atmosphere continuous decrease in energy results in anas vapor, with the soil playing a key role energy continuum from the top to the bottom ofi n water retention and storage. The remaining the falls. As an analogy, when wet soil dries, there30 percent of the precipitation represents the i s a continuous decrease in the energy content ofl ongtime annual supply of fresh water for the remaining water. When dry soil gets wetter,use in homes, industry, and irrigated agriculture. there is a continuous increase in the energy of soilThis chapter contains important concepts and water. The continuous nature of the changes inprinciples that are essential for gaining an under- the amount of soil water, and correspondingstanding of the soils role in the hydrologic cycle changes in energy, produce the soil water energyand the intelligent management of water re- continuum.sources. 54
SOIL WATER ENERGY CONTINUUM 55Adhesion WaterWater molecules (H 2 0) are electrically neutral;however, the electrical charge within the mole-cule is asymmetrically distributed. As a result,water molecules are strongly polar and attracteach other through H bonding. Soil particles havesites that are both electrically negative and posi-tive. For example, many oxygen atoms are ex-posed on the faces of soil particles. These oxygenatoms are sites of negativity that attract the posi-tive poles of water molecules. The mutual attrac-tion between water molecules and the surfaces ofsoil particles results in strong adhesive forces. If a FIGURE 5.1drop of water falls onto some oven dry soil, the Schematic drawing of relationshipwater molecules encountering soil particles will between various forms of soil water. Adhesion waterbe strongly adsorbed and the molecules will is strongly absorbed and very immobile andspread themselves over the surfaces of the soil unavailable to plants. The gravitational water, byparticles to form a thin film of water. This layer, or contrast, is beyond the sphere of cohesive forces, and gravity causes gravitational water to flow rapidlyfilm of water, which will be several water mole- down and out of the soil (unless downward flow iscules thick, is called adhesion water (see Figure i nhibited). The cohesion water is intermediate in5.1). properties and is the most important for plant Adhesion water is so strongly adsorbed that it growth.moves little, if at all, and some scientists believethat the innermost layer of water molecules existsi n a crystalline state similar to the structure of ice.Adhesion water is always present in field soils and hesive forces operate between water moleculeson dust particles in the air, but the water can be because of hydrogen bonding. This causes waterremoved by drying the soil in an oven. Adhesion molecules to attach themselves to the adhesionwater has the lowest energy level, is the most water, resulting in an increase in the thickness of i mmobile water in the soil, and is generally un- the water film. Water that is retained in soils be-available for use by plant roots and microor- cause of cohesive forces is called cohesion water, ganisms. and is shown in Figure 5.1. About 15 to 20 molecular layers is the maxi-Cohesion Water mum amount of water that is normally adsorbed to soil particle surfaces. With increasing distanceThe attractive forces of water molecules at the from the soil particle surface toward the outersurface of soil particles decrease inversely and edge of the water film, the strength of the forcesl ogarithmically with distance from the particle that holds the water decreases and the energy andsurface. Thus, the attraction for molecules in the mobility of the water increase. Adhesion water issecond layer of water molecules is much less than so strongly held that it is essentially immobile,for molecules in the innermost layer. Beyond the and thus is unavailable to plants. The cohesionsphere of strong attraction of soil particles for water is slightly mobile and generally available towater molecules of the adhesive water layer, co- plants. There is a water-energy gradient across the
56 SOIL WATERwater films, and the outermost water molecules Gravitational Waterhave the greatest energy, the greatest tendency to I n soils with claypans, a very slowly permeablemove, and the greatest availability for plant roots. subsoil layer permits little water, in excess of fieldAs roots absorb water over time, the soil water capacity, to move downward and out of the soil.content decreases, and water films become thin- After a long period of rainfall, water accumulatesner; roots encounter water with decreasing energy above the claypan in these soils. As the rainycontent or mobility. At some point, water may period continues, the soil above the claypan satu-move so slowly to roots that plants wilt because of rates from the bottom upward. The entire A hori-a lack of water. If wilted indicator plants are zon or root zone may become water saturated.placed in a humid chamber and fail to recover, the Soil water that exists in aeration pores, and that issoil is at the permanent wilting point (see Figure normally removed by drainage because of the5.2). This occurs when most of the cohesion wa- force of gravity, is gravitational water. When soilster has been absorbed. are saturated with water, the soil volume compo- When soil particles are close to, or touch, adja- sition is about 50 percent solids and 50 percentcent particles, water films overlap each other. water. Gravitational water in soil is detrimentalSmall interstices and pores become water filled. when it creates oxygen deficiency. GravitationalIn general, the pores in field soils that are small water is not considered available to plants be-enough to remain water filled, after soil wetting cause it normally drains out of soils within a dayand downward drainage of excess water has oc- or two after a soil becomes very wet (see Figurecurred, are the capillary or micropores. At this 5.1).point, the soil retains all possible water against The soil in the depression, of Figure 5.3 is watergravity (adhesion plus cohesion water), and the saturated because underlying soil is too slowlysoil is considered to be at field capacity. The permeable. Note the very poor growth of corn inaeration or macropores at field capacity are air the soil that remains water saturated after rainfall.filled. A typical loamy surface soil at field capacity Along the right side, the soil is not water satu-has a volume composition of about 50 percent rated. The gravitational water has drained out, thesolids, 25 percent water, and 25 percent air. aeration pores are air filled, and the soil is at field capacity. Here, the growth of corn is good. An energy gradient exists for the water within a FIGURE 5.2 Wilted coleus plant on the right. If the macropore, including the gravitational water. plant fails to revive when placed in a humid When water filled macropores are allowed to chamber, the soil is at the permanent wilting point in drain, the water in the center of the macropore has regard to this plant. the most energy and is the most mobile. This water leaves the pore first and at the fastest speed. This is followed by other water molecules closer to the pore edges until all of the gravitational water has drained, leaving the cohesion and ad- hesion water as a film around the periphery of the macropore. Summary Statement 1. The forces that affect the energy level of soil water, and the mobility and availability of wa-
ENERGY AND PRESSURE RELATIONSHIPS 57 FIGURE 5.3 Water-saturated soil occurs in the depression where corn growth is poor. The surrounding soil at higher elevation is at field capacity because of recent rain, and corn growth is good owing to high availability of both water and oxygen. ter to plants, are adhesion, cohesion, and i n the water could be used to generate electricity. gravity. The greater the water pressure, the greater the 2. A water-saturated soil contains water with tendency of water to move and do work, and the widely varying energy content, resulting in an greater the energy of the water. energy continuum between the oven dry state The next consideration will be that of water and water saturation. pressure relationships in water-saturated soils, 3. The mobility and availability of water for plant which is analogous to the water pressure relation- use parallel the energy continuum. As dry soil ships in a beaker of water or in any body of water. wets over time, the soil water increases in Pressure Relationships in Saturated Soil energy, mobility, and availability for plants. As wet soil dries over time, the energy, mobil- i ty, and plant availability of the remaining wa- The beaker shown in Figure 5.4 has a bottom area ter decreases. assumed to be 100 cm² . The water depth in the 4. Plants wilt when soil water becomes so im- beaker is assumed to be 20 centimeters. The vol- mobile that water movement to plant roots is ume of water, then, is 2,000 cm ³ and it weighs too slow to meet the transpiration (evapora- 2,000 grams. The pressure, P, of the water at the tion from leaves) needs of plants. bottom of the beaker is equal to force = 2,000 g P= = 20 g/cm ² area 100 cm² ENERGY AND PRESSURE RELATIONSHIPS The water pressure at the bottom of the beaker There is a relationship between the energy of wa- could also be expressed simply as equal to a ter and the pressure of water. Because it is much column of water 20 centimeters high. This is anal- easier to determine the pressure of water, the ogous to saying that the atmospheric pressure at energy content of water is usually categorized on sea level is equal to I atmosphere, is equivalent to the basis of pressure. a column of water of about 33 feet, or to a column The hull of a submarine must be sturdily built to of mercury of about 76 centimeters. These are withstand the great pressure encountered in a also equal to a pressure of 14.7 lb/in. ² dive far below the oceans surface. The column At the 10-centimeter depth, the water pressure (head) of water above the submarine exerts pres- i n the beaker is one half of that at the 20- cen- sure on the hull of the submarine. If the water timeter depth and, therefore, is 10 g/cm ² . The pressure exerted against the submarine was di- water pressure decreases with distance toward rected against the blades of a turbine, the energy the surface and becomes 0 g/cm ², at the free
58 SOIL WATER open water surface and moving upward into a capillary tube, water pressure continues to de- crease. Thus, the water pressure in the capillary tube is less than zero, or is negative. As shown in Figure 5.5, the water pressure decreases in the capillary tube with increasing height above the open water surface. At a height of 20 centimeters above the water surface in the beaker, the water pressure in the capillary is equal to -20 g/cm ² . Figure 5.5 also shows that at this same height above the water surface, the water pressure in unsaturated soil is the same at the 20-centimeter height. The two pressures are the same at any one height, and there is no net flow of water from the soil or capillary tube after an equilibrium condi- A beaker with a cross-sectional area of tion is established. 100 square centimeters contains 2,000 grams of waterFIGURE 5.4 Applying the consideration of water in a capil-when the water is 20 centimeters deep. At the water l ary tube to an unsaturated soil, allows the follow-surface, water pressure is 0; the water pressure i ng statements:increases with depth to 20 grams per squarecentimeter at the bottom. Water pressure in a capillary tubewater surface, as shown in Figure 5.4. In water- FIGURE 5.5 decreases with increasing distance above the watersaturated soil, the water pressure is zero at the i n the beaker. It is -20 grams per square centimetersurface of the water table and the water pressure at a height 20 centimeters above the water surface.i ncreases with increasing soil depth. Since the water column of the capillary tube is continuous through the beaker and up into the soil column, the water pressure in the soil 20 centimeters above the water level in the beaker is also -20 grams per square centimeter.Pressure Relationships inUnsaturated SoilIf the tip of a small diameter glass or capillary tubeis inserted into the water in a beaker, adhesioncauses water molecules to migrate up the interiorwall of the capillary tube. The cohesive forcesbetween water molecules cause other water mol-ecules to be drawn up to a level above the water inthe beaker. Now, we need to ask: "What is thepressure of the water in the capillary tube?" and"How can this knowledge be applied to unsatu-rated soils?" As shown previously, the water pressure de-creases from the bottom to the top of a beakerfilled with water and, at the top of the water sur-face, the water pressure is zero. Beginning at the
THE SOIL WATER POTENTIAL 591. Water in unsaturated soil has a negative pres- reference pool (which is at a lower elevation) to sure or is under tension. soil that is at a higher elevation, the sign is posi-2. The water pressure in unsaturated soil de- tive. The higher the water above the reference creases with increasing distance above a free point, the greater the gravitational water potential. water surface or water table. Visualize a water-saturated soil that is under- lain at the 1-meter depth with plastic tubing con- taining small holes. Normally, the plastic tubingTHE SOIL WATER POTENTIAL allows gravitational water to drain out of the soil.Most of the concerns about soil water involve Suppose, however, that the tube outlet is closedmovement: water movement into the soil surface, and the soil remains water saturated. The gravita-water movement from the soil surface downward tional water potential at the top of the soil that isthrough soil, and movement of water from the soil saturated, relative to the plastic tubing, is equal toto, and into, roots, microorganisms, and seeds. a 1-meter water column. This is the same as say-The pressure of soil water, which is an indication i ng that the water pressure is equal to a 1-meterof tendency for soil water to move, is expressed by column of water in reference to the point of waterthe soil water potential. Technically, the soil wa- drainage through the tubing.ter potential is defined as the amount of work that If the drainage tube outlet is opened, gravita-must be done per unit quantity of water to trans- tional water will leave the water-saturated soil andport or move reversibly (without energy loss due drain away through the plastic tubing. The soilto friction) and isothermally (without energy loss will desaturate from the surface downward. Thedue to temperature change) an infinitesimal water pressure or gravitational potential at the topquantity of water from a pool of pure water at of water-saturated soil will decrease as the waterspecified elevation and at atmospheric pressure l evel drops. When the gravitational water hasto the soil water at the point under consideration. been removed, and drainage stops, the top of theIt is important to note that the water potential is new level of water-saturated soil will be at thethe amount oph work needed to move water phrom a same elevation as the drainage tubing (the refer- ence point). At this elevation, the gravitationalrepherence pool to another point. potential will be zero. The symbol for the water potential is psi, 0. The The Matric Potentialtotal water potential, q t , i s made up of severalsubpotentials, including the gravitational, matric,and osmotic. When a soil is unsaturated and contains no gravi- tational water, the major movement of water isThe Gravitational Potential l aterally from soil to plant roots. The important forces affecting water movement are adhesionThe gravitational potential, g , is due to the and cohesion. Adhesion and cohesion effects areposition of water in a gravitational field. The gravi- i ntimately affected by the size and nature of pri-tational potential is very important in water- mary soil particles and peds. The resulting physi-saturated soils. It accounts primarily for the move- cal arrangement of surfaces and spaces, owing toment of water through saturated soils or the texture and structure, is the soil matrix. The inter-movement of water from high to low elevations. action of the soil matrix with the water producesCustomarily, the soil water being considered is the matric water potential. The matric potentialhigher in elevation than the reference pool of pure i s 1m .water. Since the gravitational potential represents When rain falls on dry soil, it is analogous tothe work that must be done to move water from a moving water molecules from a pool of pure water
60 SOIL WATERto the water films on soil particles. No energy osmotic potential has little, if any, effect on wateri nput is required for the movement of water in movement within the soil, since diffusion of theraindrops to water films around dry soil particles. hydrated ions creates a uniform distribution andI n fact, a release of energy occurs, resulting in a the hydrated ions do not contribute to a differen-decrease in the potential of the water. Adsorbed tial in matric potential in one region of the soil aswater has lower potential to do work than water in compared to another.raindrops. In other words, negative work is re- Measurement and Expression ofquired to transfer water from raindrops to the Water Potentials films on soil particles. This causes the matric potential to have a negative sign. The matric potential varies with the content of Since the gravitational potential is the distance soil water. The drier a soil is, the greater is the between the reference point and the surface of the tendency of the soil to wet and the greater is the water-saturated soil, or the water table, it can be release of energy when it becomes wetted. In fact, measured with a ruler. Reference has been madewhen oven dry clay is wetted, a measureable in- to a gravitational potential equal to a meter-long crease in temperature occurs due to the release of water column. Water potentials are frequently ex- energy as heat. This release of heat is called the pressed as bars that are approximately equal to heat oph wetting. Generally, the drier a soil is, the atmospheres. A bar is equivalent to a 1,020 cen- l ower is the matric potential. The number ti meter column of water. preceded by a negative sign becomes larger (be- The matric potential can be measured in sev- comes more negative) when a soil dries from field eral ways. Vacuum gauge potentiometers consist capacity to wilt point. In reference to the previous of a rigid plastic tube having a porous fired clay situation where the gravitational water had just cup on one end and a vacuum gauge on the other. exited a soil with drainage tubing at a depth of 1 The potentiometer is filled with pure water, and at meter, the matric water potential in unsaturated this point the vacuum gauge reads zero. The soil at the soil surface is equal to a -1-meter potentiometer is then buried in the soil so that the column of water. porous cup has good contact with the surround- i ng soil (see Figure 5.6). Since the potential ofThe Osmotic Potential pure water in the potentiometer is greater than water in unsaturated soil, water will move fromThe osmotic potential, o, is caused by the forces the potentiometer into the soil. At equilibrium, the i nvolved in the adsorption of water molecules by vacuum gauge will record the matric potential.i ons (hydration) from the dissolution of soluble Vacuum gauge potentiometers work in the rangesalt. Since the adsorption of water molecules by 0 to -0.8 bars, which is a biologically importanti ons releases heat (energy of hydration), the sign range for plant growth. They are used in irrigatedof the osmotic potential is also negative. agriculture to determine when to irrigate. The osmotic potential is a measure of the work A pressure chamber is used to measure matricthat is required to pull water molecules away from potentials less than -0.8 bars. Wet soil is placedhydrated ions. Normally, the salt content of the on a porous ceramic plate with very fine pores,soil solution is low and the osmotic potential has which is confined in a pressure chamber. Air pres-little significance. In soils containing a large sure is applied, and the air forces water throughamount of soluble salt (saline soils), however, the the fine pores in the ceramic plate. At equilibrium,osmotic potential has the effect of reducing water when water is no longer leaving the soil, the airuptake by roots, seeds, and microorganisms. The pressure applied is equated to the matric poten-
SOIL WATER MOVEMENT 61 FIGURE 5.6 Soil water potentiometers for measuring the matric potential. The one on the left has been removed from the soil to show the porous clay cup.tial. At equilibrium, an air pressure of 15 bars is of water flow is directly related to the water poten-equal to a matric potential of -15 bars. This tech- tial difference and inversely related to the flownique is used to determine the matric potential in distance.the range of -0.3 to -15 bars, which is the range The velocity of water flow is also affected by thebetween field capacity and the permanent wilting soils ability to transmit water or the hydraulicpoint, respectively. conductivity (k). Water flow through large pores is Many books and publications express the water faster than through small pores; flow is fasterpotential in bar units. However, kilopascals and when the conductivity is greater. Therefore, themegapascals are being used more frequently to velocity of flow, V, is equal to the water potentialexpress the water potential. A bar is equivalent to gradient times the hydraulic conductivity, k. Math- 100 kilopascals (kPa). Using kilopascals, the wa- ematically,ter potential at field capacity is -30 kPa and at the V = kphpermanent wilting point is -1,500 kPa. The hydraulic conductivity, or the ability of the soil to transmit water, is determined by the natureSOIL WATER MOVEMENT and size of the spaces and pores through which the water moves. The conductivity of a soil isWater movement in soils, and from soils to plant analogous to the size of the door to a room. As theroots, like water cascading over a dam, is from door to a room becomes larger, the velocity o1regions of higher-energy water to regions of lower- speed at which people can go in or out of theenergy water. Thus, water runs downhill. room increases. As long as the door size remain: The driving force for water movement is the constant, the speed with which people can movedifference in water potentials between two points. i nto or out of the room remains constant. As longThe water potential gradient (t) is the water as the physical properties of a soil (includingpotential difference between two points divided water content) remain constant, k is constant.by the distance between the two points. The rate Conductivity (k) is closely related to pore size
62 SOIL WATERWater flow in a pipe is directly related to the fourth This equation is named after Darcy, who stud-power of the radius. Water can flow through a i ed the flow of water through a porous mediuml arge pore, having a radius 10 times that of a small with a setup like that shown in Figure 5.7. Thepore, 10,000 times faster than through the small setup includes a column in which a 12-pore. In water-saturated soils, water moves much centimeter-thick layer of water rests on top of amore rapidly through sands than clays because 15-centimeter-thick layer of water-saturated soil.sands have larger pores. The larger pores give the I n this case, the potential difference between thesandy soils a greater ability to transmit water or a two points is equal to 12 centimeters (point A togreater conductivity when saturated as compared point B) plus 15 centimeters (point B to point C)to clayey soils. or 27 centimeters; h equals 27 centimeters. The The size of pores through which water moves distance of flow through soil, d, is 15 centimeters.through soil, however, is related to the water con- If the amount of water that flowed through the soiltent of the soil. In saturated soil, all soil pores are i n 1 hour was 360 cm ³ , and the cross-sectionalwater filled and water moves rapidly through the area of the soil column is 100 cm ² , k can be largest macropores. At the same time, water calculated as follows: movement in the smaller pores is slow or nonexis- 27 cmtent. As the water content of soil decreases, water 360 cm ³ = k x x 100 cm² x I hr 15 cmmoves through pores with decreasing size, be-cause the largest pores are emptied first, followed Then, k = 2 cm/hrby the emptying of pores of decreasing size. The value fork of 2 cm/hr is in the range of soilsTherefore, k (hydraulic conductivity of the soil) is with moderate saturated hydraulic conductivity;closely related to soil water content; k decreases i n the range of 0.5 cm/hr to 12.5 cm/hr. Soils withwith decreasing soil water content. slow saturated hydraulic conductivity range from less than 0.125 cm/hr to 0.5 cm/hr. Soils withWater Movement in Saturated Soil rapid saturated hydraulic conductivity have a con- ductivity in the range 12.5 cm/hr to more thanAs shown previously, the potential gradient (ph) is FIGURE 5.7 A setup similar to that which Darcythe difference in water potentials between the twopoints of flow divided by the distance between the used to study water movement in porous materials.points of flow. The difference in water potentialsbetween two points in water-saturated soil is thehead (h). The gradient, f, is inversely related to thedistance of flow through the soil, d. Therefore,
SOIL WATER MOVEMENT 6325 cm/hr. Thus, a sand soil could have a saturated loss of water by evaporation. As a consequence,hydraulic conductivity that is several hundred gravity plays a minimum role in the movement ofti mes greater than that of a clay soil. water in water-unsaturated soil. According to Darcys equation, if the distance Darcys law also applies to the movement ofof water movement through soil is doubled, the water in unsaturated soil. The value for h, thequantity of water flow is halved. This is an impor- hydraulic head, becomes the difference in thetant consideration in soil drainage, because the matric potentials between the points of flow. Forfurther soil is from the drain lines, the longer it water movement from soil to roots, h will com-will take for water to exit the soil. Therefore, the monly be a few hundred kPa (a few bars);distance between drainage lines is dependent on for example, the difference between -100 andthe hydraulic conductivity. Another important de- -300 kPa (-1 to -3 bars).cision based on the saturated hydraulic conduc- A most important consideration for unsaturatedtivity includes determining the size of seepage flow is the fact that unsaturated hydraulic conduc-beds for septic tank effluent disposal. tivity can vary by a factor of about 1 million within the range that is biologically important. Unsatu-Water Movement in Unsaturated Soil rated hydraulic conductivity decreases rapidly when plants absorb water and soils become drierIf water is allowed to drain from a saturated soil, than field capacity. For the Geary soil (see Tablewater leaves the soil first and most rapidly via the 5.1) , k near field capacity at -33 kPa is 1.1 x 10 - largest pores. As drainage proceeds over time, compared with 6.4 x 10 -5 cm/day for a potentialwater movement shifts to smaller and smaller of -768 kPa. This means that the conductivity inpores. The result is a rapid decrease in both hy- the soil at field capacity is more than 1,700 timesdraulic conductivity and in the rate of drainage, or greater than in soil midway between field capacitymovement of water from the soil. Field soils with and the permanent wilting point. The water inan average saturated hydraulic conductivity will, unsaturated soil is very immobile and significantessentially, stop draining within a day or two after movement occurs only over very short distances.a thorough wetting, meaning the gravitational wa- The distance between roots must be small to re-ter has exited the soil and the soil is retaining the cover this water. It is not surprising that Dittmermaximum amount of water against the force of (1937) found that a rye plant, growing in a cubicgravity. Such soil in the field is at field capacity foot of soil, increased root length about 3 milesand will have a matric potential varying from per day when consideration was made for rootabout -10 to -30 kPa (-0.1 to -0.3 bars). The hair growth. When plants wilt for a lack of water,data in Table 5.1 show that the conductivity of the the wilting is due more to the slow transmissionGeary silt loam decreased from 9.5 cm/day at rate of water from soil to roots rather than a lowwater-saturation (matric potential 0) to only water content of the soil per se. 1.1 x 10 - near field capacity (matric potential Water Movement in Stratified Soilequal to -33 kPa or -0.3 bars). Thus, the hydrau-lic conductivity at field capacity was about 1 per-cent as great as at saturation. Stratified soils have layers, or horizons, with dif- Gravity is always operating. After field capacity ferent physical properties, resulting in differencesis attained, however, matric forces control water i n hydraulic conductivity. As a consequence, themovement. Additional movement is very slow and downward movement of water may be alteredover short distances. The flow tends to be hori- when the downward moving water front encoun-zontally from soil to roots, except for some verti- ters a layer with a different texture.cal movement at the soil surface, resulting from When rain or irrigation water is added to the
64 SOIL WATER TABLE 5.1 Volume Water Content, Hydraulic Conductivities, and Matric Potentials of a Loam and Silt Loam Soil Data from Hanks, 1965.surface of dry, unsaturated soils, the immediate If the water is moving through loam soil and thesoil surface becomes water saturated. Infiltration, wetting front encounters a dry sand layer, the wet-or water movement through the soil surface, is ting front stops its downward movement. The wet-generally limited by the small size of the pores in ting front then continues to move laterally abovethe soil surface. Water does not rush into the the sand layer, as shown in Figure 5.8, becauseunderlying drier unsaturated soil, but moves the unsaturated hydraulic conductivity in the dryslowly as unsaturated flow. The downward flow of sand is less than that of the wet silt loam soilwater below the soil surface is in the micropores, above. Water in the wetting front is moving as awhile the macropores remain air-filled. The water liquid and, in dry sand, there are essentially nomoves slowly through the soil as a wetting front, continuous water films between sand grains. Thatcreating a rather sharp boundary between moist is, the path of water movement is interrupted be-and dry soil. Similarly, water moves slowly from cause the water films in many adjacent sandan irrigation furrow in all directions. Matric forces grains do not touch each other. The abrupt stop- (cohesion and adhesion) control water move- page of the water front, shown in Figure 5.8, isment; water tends to move upward almost as rap- analagous to a vehicle moving at a high speed andi dly as downward. abruptly stopping when it reaches a bridge that
SOIL WATER MOVEMENT 65 FIGURE 5.8 Photographs illustrating water movement in stratified soil where water is moving as unsaturated flow through a silt l oam into a sand layer. In the upper photograph, water movement into the sand is inhibited by the low unsaturated hydraulic conductivity of dry sand. The lower photograph shows that after an elapsed time of 1.5 to 5 hours, the water content in the lower part of the silt loam soil became high enough to cause water drainage downward into the sand. (Photographs courtesy Dr. W. H. Gardner, Washington State University.)has collapsed. There is a lack of road continuity any sand and gravel layers underlying loamy soils,where the bridge has collapsed. The distance be- enables the loam soil layer to retain more watertween particles in clay soils is small, and there is than if the the soil had a loam texture throughout.greater likelihood of water films bridging between A gravel layer under golf greens results in greaterthe particles in dry clay soil as compared to dry water storage in the overlying soil. Conversely,sand. This gives dry clay soils greater hydraulic during periods of excessive rainfall, drainage willconductivity than dry sands. occur and prevent complete water saturation of As the soil under the irrigation furrow, and the soil on the green.above the sand layer, becomes wetter over time, If a clay layer underlies a loamy soil, the wettingthe water potential gradient between the moist front will immediately enter the clay. Now,soil and dry soil increases. In time, the water however, the clays capacity to transmit water isbreaks through the silt loam soil and enters the very low by comparison, and water will continuesand layer, as shown in the lower part of Figure to move downwward in the loam soil at a faster5.8. The low unsaturated hydraulic conductivity of rate than through the clay layer. Under these con-
66 SOIL WATERditions, the soil saturates above the clay layer, as PLANT AND SOIL WATER RELATIONSit does when a claypan exists in a soil. From this Plants use large quantities of water, and this sec-discussion it should be apparent that well-drained tion considers the ability of plants to satisfy theirsoils tend never to saturate unless some underly- water needs by absorbing water from the soil andi ng layer interrupts the downward movement of a the effect of the soil water potential on nutrientwetting front and soil saturates from the bottom uptake and plant growth.upward. Available Water Supplying Power At the junction of soil layers or horizons of of Soilsdifferent texture, downward moving water tendsto build-up and accumulate. This causes a tempo-rary increase in the availability of water and of The water-supplying power of soils is related tonutrients, unless very wet and saturated soil is the amount of available water a soil can hold. Theproduced, and results in increased root growth available water is the difference in the amount ofand uptake of nutrients and water near or at these water at field capacity (-30 kPa or -0.3 bar) andj unctions. the amount of water at the permanent wilting point (-1,500 kPa or -15 bars).Water Vapor Movement The amount of available water is related to both texture and structure, because it is dependent onWhen soils are unsaturated, some of the pore the nature of the surfaces and pores, or soil ma-space is air filled and water moves through the trix. Soils high in silt (silt loams) tend to have thepore space as vapor. The driving force is the dif- most optimum combination of surfaces andference in water potentials as expressed by differ- pores. They have the largest available water-ences in vapor pressure. Generally, vapor pres- holding capacity, as shown in Figure 5.9. Thesesure is high in warm and moist soil and low in soils contain about 16 centimeters of plant-cold and dry soil. Vapor flow, then, occurs from available water per 100 centimeters of depth, or 16warm and moist soil to and into cold and dry percent by volume. This is about 2 inches of watersoil. In the summer, vapor flow travels from the per foot of soil depth. When such soil is at thewarmer upper soil horizons to the deeper and permanent wilting point, a 2-inch rain or irri-colder soil horizons. During the fall, with transi- gation, which infiltrates completely, will bring thetion to winter, vapor flow tends to be upward. It soil to field capacity to a depth of about 1 foot.i s encouraged by a low air temperature and up- From Figure 5.9, it can be noted that sands haveward water vapor movement, and condensation the smallest available water-holding capacity.onto the surface of the soil commonly creates This tends to make sands droughty soils. Anothera slippery smear in the late night and early morn- reason why sandy soils tend to be droughty is thati ng hours. most of the available water is held at relatively Soil conductivity for vapor flow is little affected high water potential, compared with loams andby pore size, but it increases with an increase in clays. It should be noted in Figure 5.10 that fieldboth total porosity and pore space continunity. capacity for sand is more likely nearer -10 kPaWater vapor movement is minor in most soils, but (-0.1 bar) than -30 kPa (-0.3 bar) as in loamyit becomes important when soils crack at the sur- and clayey soils. This means that much of theface in dry weather and water is lost from deep plant available water in sandy soils can movewithin the soil along the surfaces of the cracks. rapidly to roots when water near roots is depleted,Tillage that closes the cracks, or mulches that because of the relatively high hydraulic conduc- cover the cracks, will reduce water loss by evapo- tivity at these relatively high potentials. Plants ration. growing on clay soils, by contrast, may not absorb
PLANT AND SOIL WATER RELATIONS 67 Relationship of soil texture to available water-holding FIGURE 5.9 capacity of soils. The difference between the water content at field capacity and the water content at the permanent wilting point is the available water content.as much water as they could use during the day sands tend to be more droughty than clays be-because of lower hydraulic conductivity and cause they retain less water at field capacity, andslower replacement of water near roots when wa- the water retained is consumed more rapidly.ter is depleted. This results in a tendency to con-serve the available water in fine-textured soils. Inessence, the water retained at field capacity in Water Uptake from Soils by Rootssands is more plant available than the water in Plants play a rather passive role in their use offine-textured soils at field capacity. Therefore, water. Water loss from leaves by transpiration is FIGURE 5.10 Soil water characteristic curves for three soils. Field capacity of the sandy loam is about -0.1 kPa x 10 -2 (-10 kPa) and for the finer-textured soils is about -0.3 kPa x 10 -2 (-30 kPa).0
68 SOIL WATERmainly dependent on the surrounding environ- Diurnal Pattern of Water Uptakement. Water in the atmosphere has a much lower Suppose that it is a hot summer morning, andwater potential than water in a leaf, causing the there has been a soaking rain or irrigation duringatmosphere to be a sink for the loss of water from the night. The soil is about at field capacity andthe plant via transpiration. The water-conducting water availability is high. The demand for water istissue of the leaves (xylem) is connected to the nil, however, because the water-potential differ-xylem of the stem, and a water potential gradient ences in the leaves, roots, and soil are small.develops between leaves and stems due to water There is no significant water gradient and no wa-l oss by transpiration. The xylem of the stem is ter uptake. After the sun rises and the temperatureconnected to the xylem of the roots, and a water i ncreases, thus decreasing relative humidity, thepotential gradient is established between the stem water potential gradient increases between theand roots. The water potential gradient estab- atmosphere and the leaves. The leaves begin tolished between roots of transpiring plants and soil transpire. A water potential gradient is establishedcauses water to move from the soil into roots. This between the leaves, roots, and soil. Roots beginwater potential gradient, or continuum, is called water uptake. The low water conductivity in (thethe soil-plant-atmosphere-continuum, or SPAC. plant causes the water potential difference be-Some realistic water potentials in the continuum tween leaves and roots, and between roots andduring midday when plants are actively absorb- soil, to increase to a maximum at about 2 P.M., asi ng water are -50,000 kPa (-500 bars) in the shown in Figure 5.11. This is a period of greateratmosphere, - 2,500 kPa (-25 bars) in the leaf, water loss than water uptake, which creates a- 800 kPa (-8 bars) in the root, and -700 kPa water deficit in the leaves. It is normal for these(-7 bars) in the soil. There is considerable resis- leaves to be less than fully turgid and appeartance to upward water movement in the xylem, so slightly wilted.that a considerable water potential gradient can After 2 P.M., decreasing temperature and in-be produced between the leaf and the root. creasing relative humidity tend to reduce water FIGURE 5.11 Diurnal changes in water potential of soil, roots, and leaves during a period of soil drying. (After Slatyer, 1967.)
PLANT AND SOIL WATER RELATIONS 69 l oss from leaves. With continued water uptake, -6,000 kPa (-10 bars to -60 bars), which repre-the plant absorbs water faster it is lost, and the sents a rather narrow range in differences in soilwater potential gradient between leaves and soil water content. Desert plants can thrive with verydecreases. The water deficit in the plant is de- l ow water potentials and, typically, have somecreased, and any symptoms of wilting disappear. special feature that enables them to withstand a By the next morning, the water potential gradient li mited supply of soil water. Some plants shedbetween the leaves and soil is eliminated (see their leaves during periods of extreme water Figure 5.11). stress. As this pattern is repeated, the soil dries and the Pattern of Water Removal from Soilhydraulic conductivity decreases rapidly and thedistance water must move to roots increases. Inessence, the availability of soil water decreases With each passing summer day, water movesover time. As a consequence, each day the po- more slowly from soil to roots as soils dry, and thetential for water uptake decreases and the availability of the remaining water decreases. Thisplant increasingly develops more internal water encourages root extension into soil devoid ofdeficit, or water stress, during the afternoon. roots where the water potential is higher and wa-The accumulated effects of these diurnal ter uptake is more rapid. For young plants with achanges, in the absence of rain or irrigation small root system, there is a continual increase inor the elongation of roots into underlying moist rooting depth to contact additional supplies ofsoil, are shown in Figure 5.11. Eventually, available water in the absence of rain or irrigationl eaves may wilt in midday and not recover their during the summer. In this way the root zone isturgidity at night. Permanent wilting occurs, the progressively depleted of water as shown in Fig-soil is at the permanent wilting point, and the ure 5.12. When the upper soil layers are rewettedsoil has a water potential of about -1,500 kPa by rain or irrigation, water absorption shifts back(-15 bars). toward the surface soil layer near the base of the The sequence of events just described is typical plant. This pattern of water removal results in:where roots are distributed throughout the soil (1) more deeply penetrating roots in dry yearsfrom which water uptake occurs. For the se- than in wet years, and (2) greater absorption ofquence of events early in the growing season of an water from the uppermost soil layers, as com-annual plant, continual extension of roots into pared to the subsoil layers, during the growingunderlying moist soil may occur to delay the time season. Crops that have a long growing seasonwhen the plant wilts in the absence of rain or and a deep root system, like alfalfa, absorb airrigation. greater proportion of their water below 30 cen- The permanent wilting point is not actually a ti meters (see Figure 5.13).point but a range. Experiments have shown that Soil Water Potential Versussome common plants could recover from wilting Plant Growthwhen the potential decreased below -1,500 kPa (-15 bars). High temperature and strong winds,on the other hand, can bring about permanent Most plants cannot tolerate the low oxygen levelswilting at potentials greater than - 1,500 kPa. If a of water-saturated soils. In fact, water saturationplant can survive and, perhaps, make slow growth of soil kills many kinds of plants. These plantswith a small water demand, the soil water po- experience an increase in plant growth from satu- tential may be much less than -1,500 kPa. ration to near field capacity. As soils dry beyond For most soils and plant situations, the perma- field capacity, increased temporary wilting during nent wilting range appears to be about -1,000 to the daytime causes a reduction in photosynthesis.
70 SOIL WATER FIGURE 5.12 Pattern of water used from soil by sugar beets during a 4-week period following irrigation. Water in the upper soil layers was used first. Then, water was removed from increasing soil depths with time since irrigation. (Data from Taylor, 1957.)FIGURE 5.13 Percentage of water used from each Thus, plant growth tends to be at a maximum30-centimeter layer of soil for crops produced with when growing in soil near field capacity, whereirrigation in Arizona. Onions are a shallow-rooted the integrated supply of both oxygen and waterannual crop that is grown in winter and used a totalof 44 centimeters of water. Alflalfa, by contrast, grew are the most favorable.the entire year and used a total of 186 centimeters of During periods of water stress, the damage towater, much of it from deep in the soil. (Data from plants is related to the stage of development. TheArizona Agr. Exp. Sta. Tech. Bull. 169, 1965.) data in Table 5.2 show that four days of visible wilting during the early vegetative stage caused a 5 to 10 percent reduction in corn yield compared with a 40 to 50 percent reduction during the time of silk emergence and pollination. Injury from water stress at the dough stage was 20 to 30 percent. TABLE 5.2 Effect of Four Days of Visible Wilting on Corn Yield
SOIL WATER REGIME 71Role of Water Uptake forNutrient UptakeMass flow carries ions to the root surfaces, wherethey are in a position to be absorbed by roots. Ani ncreased flow of water increases the movementof nutrient ions to roots, where they are availablefor uptake. This results in a positive interactionbetween water uptake and nutrient uptake.Droughts reduce both water and nutrient uptake.Fertilizers increase drought resistance of plantswhen their use results in greater root growth androoting depth (see Figure 5.14).FIGURE 5.14 Fertilizer greatly increased the growthof both tops and roots of wheat. Greater root densityand rooting depth resulted in greater use of storedsoil water. (Photograph courtesy Dr. J. B.Fehrenbacher, University of Illinois.) FIGURE 5.15 Climatic data and average, soil water storage capacity were used to construct the water regime for soils near Memphis, Tennessee. (Data from Thornthwaite and Mather, 1955.) SOIL WATER REGIME The soil water regime expresses the gradual changes in the water content of the soil. For exam- ple, a soil in a humid region may at some time during the year have surplus water for leaching, soil water depletion by plant uptake, and water recharge of dry soil. These conditions are illus- trated in Figure 5.15 for a soil with an average water-storage capacity in a humid region, such as near Memphis, Tennessee. The early months of the year (January to May) are characterized by surplus water (see Figure 5.15). The soil has been recharged with water by
72 SOIL WATERrains from the previous fall, when the soil was differences and inversely related to the distance ofbrought to field capacity. The precipitation in the flow.early part of the year is much greater than the The rate of water flow is also directly related topotential for water evaporation (potential evapo- soil hydraulic conductivity. The hydraulic con-transpiration), and this produces surplus water. ductivity decreases rapidly as soils dry.This water (gravitational) cannot be held in a soil Plant uptake of water reduces the water contentat field capacity, and it moves through the soil, of the soil and greatly decreases the hydraulicproducing leaching. During the summer months conductivity. Thus, as soil dries, water movementthe potential evapotranspiration is much greater to roots becomes slower. Plants wilt when waterthan the precipitation. At this time, plants deplete movement to roots is too slow to satisfy the plantsstored soil water. Since there is insufficient stored demand for water.water to meet the potential demand for water During the course of the year the major changes(potential evapotranspiration), a water deficit is i n soil water content include: recharge of dry soil,produced. This means that most of the plants surplus water and leaching, and depletion ofcould have used more water than was available. stored water. Water deficits occur when waterAbundant rainfall in some years prevents a deficit; l oss from the soil by evapotranspiration exceedsi n very dry years, the water deficit may be quite the precipitation plus stored water for a period oflarge and a drought may occur. In the fall the ti me.potential evapotranspiration drops below thelevel of the precipitation, and water recharge ofdry soil occurs. Recharge, in this case, is com-pleted in the fall, and additional pecipitation con- REFERENCEStributes mainly to surplus (gravitational) water Classen, M. M. and R. H. Shaw. 1970. "Water Deficitand leaching. Leaching occurs only during the Effects on Corn:II Grain Components." Agron. J.period when surplus water is produced. 62:652-655. Dittmer, H. J. 1937. "A Quantative Study of the Roots The water regime in desert climates is charac- and Root Hairs of a Winter Rye Plant." Am. J. Bot.terized by little water storage, a very small soil Erie, L. J., 0. F. French, and K. Harris. 1965. "Con- 24:417-420.water depletion, a very large deficit, and no sur-plus water for leaching. sumptive Use of Water by Crops in Arizona." Arizona Agr. Exp. Sta. Tech. Bull. 169.SUMMARY Gardner, W. H. 1968. "How Water Moves in the Soil." Crops and Soils. November, pp. 7-12.The water in soils has an energy level that varies Hanks, R. J. 1965. "Estimating Infiltration from Soilwith the soil water content. The energy level of Moisture Properties." J Soil Water Conserv. 20:soil water increases as the water content in- 49-51.creases and decreases as the water content of the Hanks, R. J. and G. L. Ashcroft. 1980. Applied Soil Physics. Springer-Verlag, New York.soil decreases. This produces a soil water-energy Slayter, R. 0. 1967. Plant-water Relationships. Aca-continuum. demic Press, New York. The energy level of soil water is expressed as Taylor, S. A. 1957. "Use of Moisture by Plants." Soil,the soil water potential, commonly as bars or USDA Yearbook. Washington, D.C.kilopascals. Thornthwaite, C. W. and J. R. Mather. 1955. "Clima- Water moves within soil and from soil to roots tology and Irrigation Scheduling." Weekly Weatheri n response to water potential differences. Water and Crop Bulletin. National Summary of June 27,movement is directly related to water potential 1955.
CHAPTER 6 SOIL WATER MANAGEMENTThe story of water, in a very real sense, is the story subhumid climates. Techniques for water conser-of humankind. Civilization and cities emerged vation are designed to increase the amount ofalong the rivers of the Near East. The worlds water that enters the soil and to make efficient useoldest known dam in Egypt is more than 5,000 of this water.years old. It was used to store water for drinking Modifying the Infiltration Rateand irrigation, and perhaps, to control flood-waters. As the worlds population and the need for Infiltration is the downward entry of water throughfood and fiber increased, water management be- the soil surface. The size and nature of the soilcame more important. Now, in some areas of pores and the water potential gradient near therapidly increasing urban development and limited soil surface are important in determining thewater supply, competition for the use of water has amount of precipitation that infiltrates and thearisen between agricultural and urban users. amount that runs off. High infiltration rates, there- There are three basic approaches to water man- fore, not only increase the amount of water storedagement on agricultural land: (1) conservation for plant use but also reduce flooding and soilof natural precipitation in subhumid and arid erosion.regions, (2) removal of water from wetlands, and Surface soils protected by vegetation in a natu-(3) addition of water to supplement the amount of ral forest or grassland tend to have a higher in-natural precipitation (irrigation). Two important filtration rate during rains than exposed soils inneeds for water management in urban areas in- cultivated fields or at construction sites. In anclude land disposal of sewage effluent and pre- experiment in which excessive water was applied,scription athletic turf. the protection of the soil surface by straw or bur-WATER CONSERVATION lap maintained high and constant infiltration rates, as shown in Figure 6.1. When the straw andWater conservation is important in areas in which burlap were removed, the infiltration rates de-l arge water deficits occur in soils within arid and creased rapidly. The impact of raindrops on ex- 73
74 SOIL WATER MANAGEMENT faster than the same volume of smaller pores. In natural forest and grassland areas, an abundant food supply at the soil surface encourages earth- worms, resulting in large surface soil burrows that are protected from raindrop impact by the vegeta- tive cover. I n fields, it is common for infiltration rates to decrease over time during rainstorms. This is brought about by a combination of factors. First, aFIGURE 6.1 Effect of straw and burlap and their reduction in hydraulic conductivity occurs at theremoval on soil infiltration rate. (Source: Miller and soil surface, owing to physical changes. Second,Giffore, 1974.) as the soil becomes wet, and the distance of water movement increases to wet the underlying dry soil, the velocity of water flow decreases. In-posed soil breaks up soil peds and forms a crust. filtration rates from irrigation furrows, however, aThe average pore size in the surface soil de- few hours after the start of irrigation have beencreases, causing a reduction in the infitration rate. caused by earthworms that moved toward theI nfiltration is also decreased by overgrazing and moist soil and entered the waterfilled furrows,deforestation, or by any practices that remove leaving behind channels that increased infil-plant cover and/or cause soil compaction. The tration.bare field in Figure 6.2 shows the detrimental Many farmers modify the soil surface condi-effects of raindrop impact on soil structure at the tions with contour tillage and terraces. Thesesoil surface, decreased water infiltration and, practices temporarily pond the water and slowthen, increased water runoff and soil erosion. down the rate of runoff, which results in a longer Since water movement into and through a soil ti me for the water to infiltrate the soil. In somepore increases exponentially as the radius in- situations, where there is little precipitation, run-creases, a single earthworm channel that is open off water is collected for filling reservoirs. Then,at the soil surface may transmit water 100 times surface soils with zero infiltration rates are desir- FIGURE 6.2 Effects of raindrop impact on soil structure at the soil surface and on water infiltration and water runoff. The soil is unprotected by vegetation or crop residues, resulting in a greatly reduced infiltration rate and greatly increased runoff and soil erosion. Such effects accentuate the differences in the amount of water stored in soils for plant growth on the ridge tops, side slopes, and in the low areas where runoff water accumulates and sedimentation of eroded soil occurs.
WATER CONSERVATION 75able. In these cases, the important product pro- yellow-colored strips where wheat was produced,duced by the soil is runoff water. as shown in Figure 6.3. In order to explain how water storage occurs in fallowed land, both evap-Summer Fallowing oration of water from the soil surface and water movement within the soil must be considered.Most of the worlds wheat production occurs in After a wheat crop has been harvested, the wa-regions having a subhumid or semiarid climate. ter storage period begins. The surface soil layer isI n the most humid parts of these regions, soils air dry and the underlying soil is at the approxi-have enough stored water (water from precipi- mate permanent wilting point (see Figure 6.4).tation in excess of evaporation during the winter Rain on dry soil immediately saturates and in-and early spring), plus that from precipitation dur- filtrates the soil surface, and water moves down-i ng the growing season, to produce a good crop of ward as a wetting front, similar to water movingwheat in almost every year. In the drier part of out of an irrigation furrow as shown in Figure 5.8.these regions, the water deficit is larger and in The depth of water penetration depends on themost years insufficient water is available to pro- amount of infiltration and the soil texture. Eachduce a profitable crop. Here, a practice called centimeter of water that infiltrates will moistensummer phallowing is used. Summer fallowing about 6 to 8 centimeters of loamy soil, which ismeans that no crop is grown and all vegetative near the permanent wilting point. A heavy rain-growth is prevented by shallow tillage or herbi- storm will produce an upper soil layer at fieldcides for a period of time, in order to store water capacity, whereas the underlying soil remains atfor use by the next crop. the wilting point (see Figure 6.4). After wheat is harvested, the land is left un- After the rain, some water will evaporate fromplanted. No crop is grown and weeds are con- the soil surface. This creates a water potentialtrolled by shallow cultivation or herbicides to gradient between the surface of the drying soilminimize the loss of water by transpiration. Dur- and underlying moist soil, and some water in thei ng the year of the fallow period, there is greater underlying moist soil will migrate upward. Thiswater recharge than if plants were allowed to grow phase of soil drying is characterized by rapid wa-and transpire. This additional stored water, to- ter loss. As drying near the soil surface occurs, agether with the next years precipitation, is used rapid decrease in hydraulic conductivity occurs atby the wheat. Fallowing produces a landscape of the soil surface. Water migration upward is greatlyalternating dark-colored fallow strips of soil and reduced, and the evaporative demand at the sur- FIGURE 6.3 Characteristic pattern of alternate fallow and wheat strips where land is summer fallowed for wheat production.
76 SOIL WATER MANAGEMENTFIGURE 6.4 Changes in soil water during a summer fallow period for wheatproduction.face greatly exceeds the water movement upward work perfectly, some evaporation occurs eachfrom moist soil on a hot summer day. As a result, ti me the soil surface is wetted and dried, andthe immediate surface of the soil becomes air dry some runoff occurs. A good estimate is that aboutwithin a few hours and the hydraulic conductivity 25 percent of the rainfall during the fallow periodfor liquid water approaches zero. This phase is will be stored in the soil for use in crop produc-associated with a sharp decline in water loss. tion. This extra quantity of water, however, has a Once a thin layer of surface soil is air dry, and great effect on yields, as is illustrated in Table 6.1.few water films bridge particles, water can move The frequency of yields over 1,345 or 2,690 kg/from the underlying moist soil to and through the hectare (20 or 40 bushels/acre) was significantlysurface as vapor flow. Vapor movement under i ncreased by summer fallowing. Fallowing wasthese conditions is so slow as to be largely dis- also more effective at Pendleton, Oregon, wherecounted, unless cracks exist. The result is a cap- the maximum rainfall occurs in winter, unlikeping effect that protects the stored water from Akron, Colorado, and Hays, Kansas, where maxi-being lost by evaporation. When another rain oc- mum rainfall occurs in summer. Stored soil mois-curs, the surface soil is remoistened and addi- ture has been shown to be as effective as precipi-tional water moves through the previously moist- tation during the growing season for wheatened soil to increase the depth of the recharged production.soil. Repetition of this sequence of events during Saline Seep Due to Fallowinga fallow period progressively increases the depthof recharged soil and the amount of stored soilwater. The sequence of events during a fallow- Most of the worlds spring wheat is grown in sub-cropping cycle is shown in Figure 6.4. humid or semiarid regions where winters are se- Many studies have shown a good correlation vere, such as the northern plains areas of Unitedbetween water stored at planting time and grain States, Canada, and the Soviet Union. Low tem-yield. The fallowing system, however, is not 100 perature results in low evapotranspiration, whichpercent efficient. The capping feature does not increases the efficiency of water storage during
WATER CONSERVATION 77TABLE 6.1 Percentage Distribution of Wheat Yield Categories at Two Locations in the Great Plains and OneLocation in the Columbia River Basinthe fallow period. In fact, if a fallow period is spire more water per gram of plant tissue pro-followed by a wet year, surplus water may occur duced than those growing where the plant nutri-and percolate downward and out of the root zone. ents are in abundance. Since it has been shownWhere surplus water encounters an impermeable that water loss (or use) is mainly dependent onlayer, soil saturation occurs and water tends to the environment, any practice that increases themove laterally because of gravity above the imper- rate of plant growth will tend to result in produc-meable layer. This may create a spring (seepage tion of more dry matter over a given period of timespot) at a lower elevation. The evaporation of and per unit of water used. The data in Table 6.2water from the area wetted by the spring results in show that the yield of oats increased from 86 tothe accumulation of salts from the water, and an 145 kg/hectare (2.4 to 4.0 bushels/acre) for eacharea of salt accumulation called a saline seep is 2.5 centimeters of water used in conjunction withformed. The osmotic effect of soluble salt reduces more fertilizer. In humid regions, it has commonlythe soil water potential, water uptake, and wheat been observed that fertilized crops are moreyields. In some instances the soils become too drought resistant. This may be explained on thesaline for plant growth, because of very low water basis that increased top growth results in in-potential. creased root growth and root penetration, so that Control of a saline seep depends on reducing the total amount of water consumed is greater.the likelihood that surplus water will occur. Man- Fertilizer use in the subhumid region may occa-agement practices to control saline seeps in- sionally decrease yields. If fertilizer causes a cropclude: (1) reduced fallowing frequency, (2) use of to grow faster in the early part of the season, thefertilizers to increase plant growth and water use, greater leaf area at an earlier date represents aand (3) the production of deep-rooted perennial greater potential for water loss by transpiration. Ifcrops, such as alfalfa, to remove soil water. When no rain occurs, or no irrigation water is applied,alfalfa is grown and has dried out the soil, the plants could run out of water before harvest, caus-alfalfa grows very slowly, which indicates that it is i ng a serious reduction in grain yield.once again time to begin wheat production. Because water use in crop production is mainly environmentally determined, fertilizer use and theEffect of Fertilizers on Water doubling or tripling of yields has little effect onUse Efficiency the sufficiency of the water supply. In under- developed countries, large increases in cropPlants growing in a soil that contains a relatively yields can be produced without creating a needsmall quantity of nutrients grow slower and tran- for significantly more water. Conversely, if water
78 SOIL WATER MANAGEMENT TABLE 6.2 Oats Production per 2.5 Centimeters of Water Used From Hanks and Tanner, 1952. supplies become very limited, it may be desirable gravitational water from drainage ditches into to use less land and more fertilizer to obtain the higher canals for return to the sea. Now, 800 years same amount of total production with less water l ater, about one third of the Netherlands is pro- used. tected by dikes and kept dry with pumps and canals. Soil dewatering or drainage is used to l ower the water table when it occurs at or near the soil surface, or to remove excess water that has SOIL DRAINAGE accumulated on the soil surface. The use of drainage is commonplace to dewater the soil in About A.D. 1200, farmers in the Netherlands be- the root zone of crops, to dewater the soil near came engaged in an interesting water control building foundations to maintain dry basements problem. Small patches of fertile soil, affected by and for the successful operation of septic sewage tides and floodwaters, were enclosed by dikes disposal systems, and to remove excess surface and drained. Windmills provided the energy to lift water (see Figure 6.5). FIGURE 6.5 Dewatering, surface and subsurface drainage, is required to make this land suitable for both building construction and cropping. (Photograph courtesy Soil Conservation Service, USDA.)
SOIL DRAINAGE 79Water Table Depth Versus Air and This creates a zone above the capillary fringe thatWater Content of Soil has a decreasing water content and increasing airIf a well is dug and the lower part fills with water, content toward the soil surface. This capillary-the surface of the water in the well is the top of the affected zone, plus the capillary fringe, generallywater table. Water does not rise in a well above ranges in thickness from several centimeters tothe water table. In the adjacent soil, however, several meters. For all practical purposes, soilcapillarity causes water to move upward in soil more than 2 meters above the water table ispores. A water-saturated zone is created in the soil scarcely affected by it. This means that mostabove the water table, unless the soil is so gravelly plants, even those in humid regions, do not useor stony that capillarity does not occur. The water- water from the water table.saturated zone above the water table is the capil- Generally, the soil above the capillary fringelary fringe and is illustrated in Figure 6.6. will have a water content governed by the balance Soils typically have a wide range in pore sizes. between infiltration and loss by evapotran-Some pores are large enough so that they are spiration and percolation. Soils in deserts andair-filled above the capillary fringe, depending on semiarid regions typically have a permanently drypore size and height above the capillary fringe. zone between the root zone and the water table. FIGURE 6.6 Generalized relationship of the water table to the soil water zones, and the air and water content of soil above the water table. The capillary fringe is water saturated. Upward from the capillary fringe the water content of the soil decreases and the air content increases within the capillary fringe affected zone.
80 SOIL WATER MANAGEMENTBenefits of Drainage Two conditions favoring the use of surfaceRoot tips are regions of rapid cell divison and drainage are: (1) low areas that receive a largeelongation, having a high oxygen demand. Roots amount of water from the surrounding higherof most economic crop plants do not penetrate land, and (2) impermeable soils that have an in-water-saturated soil or soil that is oxygen defi- sufficient hydraulic conductivity to dispose of thecient. The major purpose of drainage in agricul- excess water by movement downward andture and forestry is lowering the water table to through the soil to drainage tubing. Surfaceincrease the plant rooting depth. drainage is favored on the Sharkey clay soils along Some of the largest increases in plant growth the lower Mississippi River, because the soil isresulting from drainage occur in forests. Drainage slowly permeable and receives water from sur-on the Atlantic Coastal Plain increased the growth rounding higher land. Sugarcane, which is widelyof pine from 80 percent to 1,300 percent (see grown and sensitive to poor aeration, is plantedTable 6.3). Few management practices are as ef- on ridges to improve aeration in the root zone.fective as drainage for increasing tree growth. Ad- It is difficult to irrigate without applying excessditional benefits of drainage in forests are easier water in many instances. Surface drainage ditchesl ogging, with less soil disturbance, and easier site are used to dispose of excess water and to preventpreparation for the next crop. saturation of the soil on the low end of irrigated Drainage of wet soil also increases the length of fields. Surface drainage is also widely used alongthe growing season in regions with low soil tem- highways and in urban areas for surface waterperature and frost hazard. Soils with a lower water control. Rapid removal of surface water from lowcontent warm more rapidly in the spring. Seed areas of golf courses and parks is essential togermination is more rapid and roots grow faster. permit their use soon after a rain.The net result is a greater potential for plant Subsurface Drainagegrowth.Surface Drainage Ditches for the removal of gravitational water can be made quickly and inexpensively. DrainageSurface drainage is the collection and removal of ditches, however, require periodic cleaning andwater from the surface of the soil via open ditches. are inconvenient for the use of machinery. There TABLE 6.3 Effect of Drainage on Mean Annual Growth of Pine on Poorly Drained Soils of the Atlantic Coastal Plain Adapted from Terry and Hughes, 1975. a Cubic meters per hectare.
SOIL DRAINAGE 81are also many situations in which ditches are not in containers is poor soil aeration caused by over-satisfactory, for example, water removal around watering. How can this be true, since mostthe basement walls of a building. flowerpots have a large drainage hole in the bot- Drain lines are installed in fields with trenching tom? Knowledge of changes in air and water con-machines. Perforated plastic tubing is very popu- tent of soil above the water table can help providel ar and is automatically laid at the bottom of a the answer.trench as the trenching machine moves across the Consider a flowerpot filled with soil that is wa-field (see Figure 6.7). Unless some unusual con- ter saturated. Water is allowed to drain out of thedition prevails, plastic tubing should be laid at l arge hole in the bottom of the pot. Surplus waterl east 1 meter deep to have a sufficiently low water will exit rapidly from the largest soil pores at thetable between the drains to permit crops to de- top of the pot and air will be pulled into these porevelop an adequate root system, as illustrated in spaces. Gradually the zone of water saturationFigure 6.8. will move progressively downward. ShortlyDrainage in Soil of thereafter, the loss of water by gravity may essen-Container-Grown Plants tially stop. At this time, some of the soil at the bottom may still be water saturated, if all the poresMany plants are grown indoors in containers. One i n the soil are sufficiently small to attract waterof the most common problems of growing plants more strongly than gravity. This creates a "capil- lary fringe" in the bottom of the pot. Then, the air and water content of the soil above the saturatedFIGURE 6.7 I nstallation of perforated plastic tubing soil at the bottom is analogous to soil above theto remove water from a field during the wet season. saturated capillary fringe. Compare the drawing inThe plastic tubing carries water to an outlet wherethe water is disposed. (Photo courtesy Soil Figure 6.9 with Figure 6.6 and note, in particular,Conservation Service, USDA.) the decrease in water and increase in air with distance upward in the soil above soil that is water saturated. If all soil pores are very small, all the soil in a flowerpot could remain saturated after watering. A good soil for container-grown plants has many pores too large to retain water against gravity. After wetting and drainage, these large pores will become air filled, even at the bottom of the con- tainer. Special attention must be given to soil used for container-grown plants. Ordinary loamy field or garden soils are unsatisfactory. A mix of one- half fine sand and one-half sphagnum peat moss, by volume, is recommended by the University of California to produce a mix with excellent physi- cal properties for plant growth. When sand and soil are mixed together, the small soil particles fill the spaces between the sand particles, producing a low-porosity mix with few large aeration pores. Placing large gravel in the bottom of a flowerpot or in a hole dug outdoors will create a barrier to the downward flow of water from an overlying finer-textured soil layer or soil mix. The gravel
82 SOIL WATER MANAGEMENT FIGURE 6.8 The effect of drainage lines in lowering the water table. The benefit of drainage is first evident directly over the drainage lines, and it gradually spreads to the soil area between them.acts in the same manner as a large hole in a i mately 11 percent of the worlds cropland is irri-container that is filled with a soil mix. A saturated gated. Some of the densest populations are sup-soil zone tends to form above the gravel layer, ported by producing crops on irrigated land, as inwhatever the depth of the gravel. the United Arab Republic (Egypt), where 100 per- cent of the cropland is irrigated. This land is lo- cated along the Nile River. More than 12 percent of the cropland, or about 24,000,000 hectaresI RRIGATION (61,000,000 acres) are irrigated in the United States. Irrigation is extremely important in in-Irrigation is an ancient agricultural practice that creasing soil productivity throughout the world.was used 7,000 years ago in Mesopotamia. Otherancient, notable irrigation systems were located Water Sourcesi n Egypt, China, Mexico, and Peru. Today, approx- Most irrigation water is surface water resultingFIGURE 6.9 Illustration of conditions in a flowerpot from rain and melting snow. Many rivers in the world have their headwaters in mountains andafter a thorough watering and drainage. At thebottom of the pot, the soil is saturated. Soil-air flow through arid and semiarid regions where thecontent increases and water content decreases with water is diverted for irrigation. Examples includedistance upward above the water-saturated layer. The the Indus River, which starts in the Himalayasoil pore spaces operate in a manner similar to a Mountains, and the numerous rivers on the west-series of capillary tubes of varying sizes. ern slope of the Andes Mountains, which flow through the deserts of Chile and Peru to the Pa- cific Ocean. Much of the water in these rivers comes from the melting of snow in the high mountains. In fact, the extent of the snowpack is measured in order to predict the stream flow and amount of water that will be available for irri- gation the next season. Many farmers who use well water have their own source of irrigation water. About 20 percent of the water used for irrigation in the United States comes from wells. I mportant Properties of Irrigated Soils Soils well suited for irrigation have an interme- diate texture to a depth of I to 2 meters and without compact or water-impermeable layers.
I RRIGATION 83Loamy soils that are underlain by coarse sands Furrow Irrigation Furrow irrigation is similarand gravel have greater retention of water in the to flood irrigation, the difference being that theoverlying soil than soils that are loamy-textured water distribution is down furrows instead of be-throughout. The texture, together with the struc- i ng allowed to flood over the land. In furrow irri-ture, should result in an infiltration rate of about gation, which is used for row crops, water is dis-0.5 to 8 centimeters per hour and a moderate tributed down between the rows. Furrows thatwater-storage capacity. The soil should not have have a slight grade or slope are made across thean injurious soluble salt content. field. The slope of the furrow bottoms depends on soil infiltration rate, which should be sufficient toWater Application Methods distribute water along the entire length of the fur- row (see Figure 6.11). Furrow irrigation is unsatis-Choice of various methods of applying irrigation factory on sand soils, because of the very highwater is influenced by a consideration of: (1) in- i nfiltration rate and limited distance of water flowfiltration rate, (2) slope and general nature of the down the furrows.soil surface, (3) supply of water and how it is Many methods are used to distribute water intodelivered, (4) crop rotation, and (5) seasonal rain- the furrows. Gated pipe is shown in Figure 6.11,fall. The methods of distributing water can be and the use of siphon tubes is shown in Figureclassified as flood, furrow, subsurface, sprinkler, 6.12. Siphon tubes require extensive manualand drip or trickle. labor. A major advantage of flood and furrow irri- gation is that the water is distributed by gravity, soFlood Irrigation Flood irrigation floods the water distribution costs tend to be low. Sprinkler Irrigation Sprinklers are widely usedland by distributing water by gravity. Water is leti nto the upper end of the field, usually from a largeditch. Sometimes the water is distributed into ba- to irrigate lawns. Sprinkler systems are versatilesins, which is a common practice for orchards, and have some special advantages. On sands,pastures, and some grain crops, including rice where high infiltration rates permit water to flow(see Figure 6.10). Flood irrigation can easily over- only a short distance down a furrow, sprinklerswater some parts of; a field while leaving some are essential to spread water uniformly over theparts underirrigated. field. On an uneven land surface, expensive land FIGURE 6.10 Flood irrigation of grapes in the Central Valley i n California.
84 SOIL WATER MANAGEMENT FIGURE 6.11 Planned land with proper grade to give uniform distribution of water down the rows. Gated pipe is being used to distribute water into the furrows.leveling and grading can be avoided by the use of pivot sprinklers are clearly visible from the air.sprinklers. The portable nature of many sprinkler Their use on many rolling and sandy soils hassystems makes them ideally suited for use where greatly increased the production of corn and otherirrigation is used to supplement natural rainfall. grain crops on the Great Plains. In some areas,When connected to soil water-measuring appa- however, underground water reservoirs are beingrati, sprinkler systems can be made automatic. depleted, and some irrigated areas are again be- Large self-propelled sprinkler systems, which ing used as grazing land and for fallowed wheatpivot in a large circle around a well, have become production.popular. Such systems can irrigate most of the Sprinkler irrigation modifies the environmentl and in a quarter section (160 acres, or 65 hec- by completely wetting plant leaves and the soiltares). In the western part of the United States, surface and by affecting the surrounding air tem-l arge green circular irrigated areas produced by perature. The high specific heat of water makes FIGURE 6.12 Use of siphon tubes to transfer water from the head ditch into furrows.
I RRIGATION 85sprinkling an effective means of reducing frost tages include adaptability of the system to veryhazard. Sprinkler irrigation has been used for frost steep land, where other methods are unsuited,protection in such diverse situations as for winter and the ability to maintain a more uniform soilvegetables in Florida, strawberries in Michigan, water potential during the growing season.and citrus in California. Potatoes develop an irregular shape and be- come less desirable if tubers alternately growSubsurface Irrigation Subirrigation is irrigat- rapidly and slowly because of large changes in available water during the period of tuberi ng by water movement upward from a water table development.l ocated some distance below the soil surface.Some areas are naturally subirrigated where watertables are within a meter or two of the soil sur- Rate and Timing of Irrigationface. This occurs in the poorly drained areas of An ideal application of water would be a sufficientthe Sandhills in north-central Nebraska. Artifical quantity to bring the soil in the root zone to fieldsubirrigation is practiced in the Netherlands, capacity. More water may result in waterlogging ofwhere subsurface drainage lines are used to drain a portion of the subsoil or in the loss of water bysoils in the wet seasons and then used during drainage. If water percolates below the root zonedrought for subirrigation. The subirrigation de- over a period of many years, the water may ac-pends on the use of drainage ditches and pumps cumulate above an impermeable layer and pro-to create and control a water table. Subirrigation duce a perched water table. Conversely, unlessworks well on the nearly level Florida coastal enough water is applied to result in some drain-plains. The sandy soils have high saturated hy- age, it is difficult to remove the salt in the irriga-draulic conductivity, and the natural water table is tion water and prevent the development of soil 1 meter or less from the soil surface much of the salinity.time. An accepted generalization is that the time to Subirrigation is more efficient in the use of wa- irrigate is when 50 to 60 percent of the plant-ter than other systems, and is adapted only for available water in the root zone has been used.special situations. In arid regions, the upward Potentiometers are used to measure the matricmovement of water and its evaporation at the soil potential in various parts of the root zone, and thesurface results in salt accumulation. Subirrigation ti ming of irrigation is based on water potentialworks best where natural rainfall leaches any salts readings. Computer programs are available forthat may have accumulated. irrigation scheduling that are based on maintain- i ng a record of gains and losses of soil water.Drip Irrigation Drip irrigation is the frequent or Farmers commonly irrigate according to the ap- pearance of the crops. Such crops as beans, cot-almost continuous application of a very small ton, and peanuts develop a dark-green color un-amount of water to a localized soil area. Plastic der moisture stress. Some species of lawn grasseshoses about 1 or 2 centimeters in diameter, with turn to an almost black-green color when wilting.emitters, are placed down the rows or around In most instances, crops should be irrigated be-trees. Only a small amount of the potential root fore marked wilting occurs. When irrigation waterzone is wetted. Roots in the localized area can is in short supply, it may be better to curtail waterabsorb water rapidly because a high water poten- use early in the season (delay irrigation), so thattial, or water availability, is maintained. A major sufficient water is available during the reproduc-advantage of drip irrigation is the large reduction tive stages of plant growth, when water stress isi n the total amount of water used. Other advan- the most injurious to crop yields.
86 SOIL WATER MANAGEMENTWater Quality Medium salinity hazard water (C2) can be usedWater in the form of rain and snow is quite pure. if a moderate amount of leaching occurs. PlantsBy the time the water has reached farm fields, with moderate salt tolerance usually can behowever, the water has picked up soluble materi- grown without special practices for salinityals (salts) from the soils and rocks over and control.through which the water moved. Four important High salinity hazard water (C3) cannot be usedfactors in determining water quality are: (1) total on soils with restricted drainage. Even with ade-salt concentration, (2) amount of sodium relative quate drainage, special management for salinityto other cations, (3) concentration of boron and control may be required. Plants with good saltother toxic elements, and (4) the bicarbonate tolerance should be selected.concentration. Very high salinity hazard water (C4) is not suit- able for irrigation under ordinary conditions, but Total Salt Concentration An acre-foot of water may be used occasionally under very special cir- cumstances. The soil must be permeable andweighs about 2,720,000 pounds. If it contained have adequate drainage. Excess irrigation water 1 ppm of salt, it would contain 2.72 pounds of salt. must be applied to bring about considerableWater containing 735 ppm of salt would contain 1 l eaching of salt, and very salt-tolerant crops ton of salt per acre-foot of water. An annual appli- should be selected. cation of 4 acre-feet of irrigation water containing Sodium Adsorption Ratio 735 ppm of salt would involve adding salt equal to4 tons per acre each year. Since such water would The water qualitybe of good quality, there is great potential for classification in Figure 6.13 is also based on thedeveloping sufficient soluble salt content in soils sodium hazard, along the left vertical side of theso as to seriously reduce soil productivity. The figure. The sodium hazard is based on the sodium l ongtime use of soils for irrigation necessitates adsorption ratio (SAR), which is the amount ofsome provision for the leaching and removal of sodium relative to other cations.salt in drainage water in order to maintain a favor- The negative electrical charge of clay and hu-able salt balance in the soil, where the salts are mus particles is neutralized by cations that are not naturally leached out of the soil. adsorbed and balance the negative charge. This The total salt concentration, or salinity, is ex- charge, which is discussed in Chapter 10, is differ-pressed in terms of the electrical conductivity of ent and much stronger than the electronegativitythe water, and it is easily and precisely deter- that attracts water molecules. When 15 percent ormined. The electrical conductivity of irrigation more of the charge is neutralized by sodiumwater is expressed as decisiemens per meter (dS/ (Na), clay and humus particles tend to dispersem);in earlier literature as millimhos per cen- and decrease the soils hydraulic conductivity.ti meter. Four water quality classes have been es- The SAR expresses the relative amount of sodiumtablished with conductivity ranging from 0.1 to i n irrigation water relative to other cations and themore than 2.25 dS/m, as shown in Figure 6.13. likelihood that the irrigation water will increaseThe salinity hazard ranges from low to very high. the amount of adsorbed sodium in the soil. The four salinity classes of water have the fol- The SAR of irrigation water is calculated asl owing general characteristics. Low salinity water follows: (C1) can be used for irrigation of most crops, with (sodium)little likelihood that soil salinity will develop. SAR = (calcium + magnesium) 1 1²Some leaching is required, but this occurs undernormal irrigation practices, except in soils with The ion concentrations, denoted by parentheses,extremely low hydraulic conductivity. are expressed in moles per liter. The classifi-
I RRIGATION 87 FIGURE 6.13 Diagram for the classification of irrigation waters. (Data from USDA Handbook 60, 1954.)cation of irrigation waters with respect to SAR is toxic. Plants have a considerable range in toler-based primarily on the effect of the exchangeable ance. The occurrence of boron in toxic concentra-sodium on the physical condition of the soil. Irri- tion in certain irrigation waters makes it necessarygation water with SAR values of 18 to 26 has a high to consider this element in assessing water qual-sodium hazard and above 26 the sodium hazard is i ty. The permissible limit of boron in severalvery high. The four SAR classes combine with the classes of irrigation water is given in Table 6.4,four classes of salinity hazard to give 16 classes of and the relative tolerance of some plants is givenwater as shown in Figure 6.13. in Table 6.5.Boron Concentration Boron is essential to Bicarbonate Concentration In waters contain-plant growth, but the amount needed is very ing high concentrations of bicarbonate ionsmall. A small amount of boron in soil may be (HC0 3 -), calcium and magnesium tend to pre-
SOIL WATER MANAGEMENT TABLE 6.4 Permissible Limits of Boron for Several Classes of Irrigation Waters in Parts per Million Sensitive Semitolerant Tolerant Boron Class Crops Crops Crops 1 <0.33 <0.67 <1.00 2 0.33-0.67 0.67-1.33 1.00-2.00 3 0.67-1.00 1.33-2.00 2.00-3.00 4 1.00-1.25 2.00-2.50 3.00-3.75 5 >1.25 >2.50 >3.75 From Agriculture Handbook 60, USDA, 1954.cipitate as carbonates as the soil solution be- In making an estimate of water quality, the ef-comes more concentrated. This reaction does not fect of salts on both the soil and the plant must bego to completion under ordinary circumstances, considered. Various factors such as drainage andbut when it does proceed, the concentrations of soil texture influence the effects on soil. The finercalcium and magnesium are reduced and the rel- the size of clay particles and the greater the ten-ative proportion of sodium in the soil solution is dency for clay particles to expand and contracti ncreased. This results in an increase in the SAR with wetting and drying, the greater is the effect ofand a decrease in water quality. a given SAR on dispersion of clay particles and TABLE 6.5 Tolerance of Plants to Boron (In each Group, the Plants First Named are Considered as Being More Tolerant, and the Last Named More Sensitive) From Agriculture Handbook 60, USDA, 1954.
I RRIGATION 89reduction in hydraulic conductivity. The ultimate 5. Over 16: Only a few very salt-tolerant cropseffect on the plant is the result of these and other yield satisfactorily.factors that operate simultaneously. Conse- The salt-tolerance figures of crops given in Ta-quently, no method of interpretation is absolutely ble 6.6 are arranged according to major crop divi-accurate under all conditions, and interpretations sions. Within each group the crops are listed inare based commonly on experience. order of decreasing salt tolerance. The EC e values given represent the salinity level at which 10, 25,Salt Accumulation and Plant Response and 50 percent decreases in yields may be ex-Soil salinity caused by the application of irrigation pected, as compared with yields on nonsalinewater usually is not a problem in humid regions soils under comparable growing conditions. Stud-where rainfall leaches the soil some time each i es of the effects of salt on plants have shownyear. Most irrigation, however, is done in low rain- significant differences in varieties of cotton, bar-fall areas where leaching is limited and salt tends ley, and smooth brome, whereas variety differ-to accumulate in irrigated soils. Today, crop pro- ences were of no consequence for green beans,duction is reduced on 50 percent of the worlds lettuce, onions, and carrots.irrigated land as a result of salinity or drainage The effect of salts on plants is mainly indirect,problems. that is, the effect of salt on the osmotic water Saline soil contains sufficient salt to impair potential. Decreasing water potential due to saltplant growth. The salts are mainly chlorides, bi- reduces the rate of water uptake by roots andcarbonates, and sulfates of sodium, potassium, germinating seeds. Thus, an increase in soil salin-calcium, and magnesium. The best method for ity produces the same net effect on water uptakeassessing soil salinity is to measure the electrical as that produced by soil drying. In fact, the effectsconductivity of the saturated soil extract, the EC e . of decreases in osmotic and matric water poten-This procedure involves preparing a water- tials appear to be additive.saturated soil paste by adding distilled waterwhile stirring the soil until a characteristic end Salinity Control andpoint is reached. A suction filter is used to obtain Leaching Requirementa sample of the extract for making an electricalconductivity measurement. The saturated-extractelectrical conductivity is directly related to the For permanent agriculture, there must be a favor-field moisture range from field capacity to the able salt balance. The salt added to soils in irri- gation water must be balanced by the removal ofpermanent wilting point; this is a good parameter salt via leaching. The fraction of the irrigationfor evaluating the effect of soil salinity on plant water that must be leached through the root zonegrowth. to control soil salinity is termed the leaching re- Salinity effects on plants, as measured by theelectrical conductivity of the saturated extract, quirement (LR). Assuming that the goal is to irri- gate a productive soil with no change in salinity,ECe , in decisiemens per meter, dS/m, are as and where soil salinity will not be affected byfollows: l eaching from rainfall or other factors, the leach-1. 0-1: Salinity effects mostly negligible. i ng requirement is the ratio of the depth of2. 2-4: Yields of very salt-sensitive crops may be drainage water, D dw , to the depth of irrigation restricted. water, D 1 :3. 4-8: Yields of many crops restricted.4. 8-16: Only salt-tolerant crops yield satisfac- = Data LR torily. D;W
90 SOIL WATER MANAGEMENT TABLE 6.6 Salt Tolerance of Crops Expressed as the EC, at 25° C for Yield Reductions of 10, 25, and 50 Percent, as Compared to Growth on Normal Soils
I RRIGATION 91 TABLE 6.6 (continued) Adapted from Agr. Information Bull. Nos. 283 and 292 and Western Fertilizer Handbook, 1975.LR, under the conditions specified (no change in lights a major disadvantage of drip irrigation: thesoil salinity over time), is also equal to method is not suitable for application of large amounts of water. Leaching of salts without pro- EC;, LR = vision for drainage in upland soils, as on the high EC dW plains of Texas, and other irrigated areas, resultsIf the electrical conductivity of the drainage water i n salt being deposited below the root zone. Most(ECdW) that can be tolerated without adversely of the irrigated land in arid regions is located inaffecting the crop is 8 dS/m: basins or alluvial valleys containing stratified soil materials. Long and continued leaching for salt ECiw LR - control commonly results in the accumulation of 8 dS/m surplus water above impermeable layers or theFor irrigation water with an electrical conductivity formation of perched water tables. When water(ECi w) of 1, 2, and 3 dS/m, LR = 0.13, 0.25, and tables rise to within 1 to 2 meters of the soil0.38, respectively. Of the irrigation water applied surface, water moves upward by capillarity at ato the soil, 13, 25, and 38 percent, respectively, rate sufficient to deposit salt on top of the soilrepresent the amounts of the water that must be from the evaporation of water, as shown in Figureleached through the soil. Under these conditions, 6.14. It is essential in these cases that subsurfacethe salt in the irrigation water is equal to the salt in drainage systems be installed to remove drainagethe drainage water and the salt content of the soil water and maintain deep water tables. Conse-remains unchanged. When the irrigation need is quently, salinity control in arid regions is depen-10 centimeters, the total water application needed dent on drainage.is calculated as follows for an LR of 25 percent: Archaeological studies in Iraq showed that the Sumerians grew almost equal amounts of wheat LR = 10 cm + 0.25 (10 cm) = 12.5 cm and barley in 3500 B.C. One thousand years later, The need for leaching soils to control salt high- only one-sixth of the grain was the less salt-
92 SOIL WATER MANAGEMENT FIGURE 6.14 Water tables (water-saturated soil) a short distance below the soil surface result in the upward movement and evaporation of water and the accumulation of salt at the soil surface. The process results in the development of saline soil.tolerant wheat; by 1700 B.C., the production of l owed, important differences can exist in the saltwheat was abandoned. This decline in wheat pro- content of soil that is immediately under the irri-duction and increase in salt-tolerant barley coin- gation furrow and the tops of beds. Downwardcided with soil salinization. Records show wide- water movement under the furrow leaches the soilspread land abandonment, and that salt and produces low salinity. At the same time, wateraccumulation in soils was a factor in the demise moves to the top of the beds and salt is depositedof the Sumerian civilization. as water evaporates. Important differences in salt Even though good irrigation practices are fol- concentrations are produced (see Figure 6.15). FIGURE 6.15 Salt accumulation on the ridges resulting from the upward movement and evaporation of water in a cotton field. Salt content under the furrows is l ess than on the ridges due to l eaching under the furrows. (Photo courtesy Soil Conservation Service, USDA.)
I RRIGATION 93Effect of Irrigation on RiverWater Quality Soil permeability, or hydraulic conductivity, has not been adversely affected by adsorbed sodium.About 60 percent of the water diverted for irri- Reclamation of saline soils requires the re-gation is evaporated or consumed. The remainder moval of salt by leaching and maintenance of theappears as irrigation return flow (which includes water table far enough below the soil surface todrainage water) and is returned to rivers for dis- minimize upward capillary water flow. Importantposal. As a consequence, the salt concentration considerations are soil hydraulic conductivity andof river water is increased. A 2- to 7-fold increase an adequate drainage system. It is not economicali n salt concentration is common for many rivers. to reclaim some saline soils that have high clayAlong the Rio Grande in New Mexico, water diver- content and a very low hydraulic conductivity.ted at Percha Dam for Rincon Valley irrigation had Several practices are recommended for usingan average of 8 milliequivalents per liter of salt saline soils. The soil should not be allowed to dryfrom 1954 to 1963. Salt content increased to 9 at completely, because the osmotic potential retards Leasburg, 13 at the American Diversion Dam, and water uptake. Sufficient water should be applied30 milliequivalents per liter at the lower end of El with each irrigation to result in leaching of some Paso Valley. Rivers serve as sinks for the deposi- salt in drainage water. Emphasis must be placedtion of salt in drainage water from irrigated land. on the selection and growth of salt-tolerant crops.The Salton Sea, whose surface is below sea level, Considerable research effort is being directed to- serves as a salt sink for the Imperial Valley in ward the development of cultivars with greater southern California. salt tolerance.Nature and Management of Saline andSodic Soils Sodic Soils Sodic soils have been called alkali soils. Sodic soils are nonsaline and have an ad-The development of saline soils is a natural sorbed or exchangeable sodium percentageprocess in arid regions where water tables are (ESP) of 15 or more. The adsorbed cations (so-near the soil surface. Large areas have become dium, calcium, magnesium, and potassium) aresaline from irrigation and from saline seeps as a considered exchangeable, because they competeresult of summer fallowing. When sodium is an for adsorption on the negatively charged soil col-important component of the salt, a significant l oids (clay and humus), are in motion, and ex-amount of adsorbed sodium may occur and may change sites with each other. The sodium ad-disperse soil colloids and develop undesirable sorption ratio (SAR) of a soil-saturated pastephysical properties. Such soils are sodic. The extract is about equal to the ESP. A SAR of 13 orquantity, proportion, and nature of salts that are more is the same as ESP of 15 or more. The pH ispresent may vary in saline and sodic soils. This usually in the range of 8.5 to 10.0. Sodium disso-gives rise to three kinds of soils: saline, sodic, and ciates from the colloids and small amounts ofsaline-sodic. sodium carbonate form. Deflocculation or dis- persion of colloids occurs, together with a breakdown of the soil structure as shown in Fig-Saline Soils Saline soils contain enough salt to ure 6.16. This results in a massive or puddled soili mpair plant growth. White crusts of salt fre- with low water infiltration and very low hydraulicquently accumulate at the soil surface. The elec- conductivity within the soil. The soil is difficult totrical conductivity of the saturated-soil extract is 4 till, and soil crusting may inhibit seedling emer-or more dS/m. The pH is 8.5, or less, and less than gence. Organic matter in the soil disperses along 15 percent of the adsorbed cations are sodium. with the clay, and the humus coats soil particles
94 SOIL WATER MANAGEMENT Saline-Sodic Soils Saline-sodic soils are char- acterized by salinity and 15 percent or more of exchangeable sodium. As long as a large quantity of soluble salts remains in the soil, the exhange- able sodium does not cause dispersion of col- loids. The high concentration of ions in the soil solution keeps the colloids flocculated. If the sol- uble salts are leached out, however, colloids may disperse with an accompanying breakdown of soil structure and loss of water permeability. The management of these soils is a problem until the excess soluble salt has been removed and the exchangeable sodium level is reduced to below 15 percent. If the soluble salts are removed by l eaching without lowering the exchangeable so- dium, saline-sodic soils may be converted in so- dic soils. Consequently, gypsum must be added to the soil at the time leaching of salts is initiated.FIGURE 6.16 Distintegrated soil structure in an WASTEWATER DISPOSALirrigated field caused by high adsorbed sodium.(Photo courtesy Soil Conservation Service, USDA.) The billions of gallons of wastewater producedto give a black color. Sodic soils may develop daily in the United States create a need for envi-when the leaching of a saline soil results in high ronmentally acceptable water disposal methods.exchangeable sodium and low exchangeable cal- The three major wastewater disposal techniquescium and magnesium. are: (1) treatment and discharge into surface wa- The basis for treatment of sodic soil is the ters, (2) treatment and reuse, and (3) land dis-replacement of exchangeable sodium with cal- posal via septic tanks and spray irrigation. Majorcium and the conversion of any sodium carbonate consideration will be given to land disposal ofi nto sodium sulfate. Finely ground gypsum (Ca- sewage effluent from septic systems and the man-S04 21 1²0) is broadcast and mixed with the soil. - - agement of municipal wastewater that results inThe calcium replaces or exchanges for the so- return of treated water to underlying aquifers fordium adsorbed on the negatively charged colloid reuse.surfaces. The amount of gypsum needed toreplace the exchangeable sodium is the gypsumrequirement. Disposal of Septic Tank Effluent The calcium sulfate also reacts with any so- People who reside beyond the limits of municipaldium carbonate that is present to form sodium sewer lines have used septic sewage disposal sys-sulfate and calcium carbonate. Then, leaching of tems for many years. The expansion of rural resi-the soil removes both the adsorbed sodium and dential areas, and the development of summerthe sodium that was in the carbonate form, as homes near lakes and rivers have greatly in-soluble sodium sulfate. Gradually, the ex- creased the need for waste disposal in rural areas.changeable sodium percentage decreases to less The sewage enters a septic tank, usually con-than 15 percent, and the soils structure and per- structed of concrete, where solid material is di-meability improve. gested or decomposed. The liquid portion, or ef-
WASTEWATER DISPOSAL 95 fluent, flows out of the top of the septic tank and holes should be uniformly spaced over the i nto drainage lines of a filter field. The drainage area selected for the filter field. Roughen lines are laid in a bed of gravel into which the the sides of the holes to remove smeared soil effluent seeps and drains through the soil, as illus- that might interfere with water entry into the trated in Figure 6.17. soil. Add 5 centimeters of sand or fine gravel The major factor influencing the suitability of to the bottom of the holes to reduce the soils soil for filter field use is the water-saturated hy- tendency to seal. draulic conductivity. The hydraulic conductivity 2. Pour enough water into each hole so that the of sands may be too high, because the sewage water is at least 30 centimeters deep. Add effluent may move so rapidly that disease organ- water as needed to keep the water depth to 30 isms are not destroyed before shallow-water sup- centimeters for at least 4 hours-preferably plies become contaminated. Soils with low hy- overnight. This wets the soil and produces draulic conductivity are not suited for septic drain results representative of the wettest season of fields, because the sewage effluent may saturate the year. soil and contaminate the surface soil. 3. The next day, adjust the water level in the test Most local regulations require that trained per- holes so that the water is 30 centimeters deep. sonnel make a percolation test before approving Measure the drop in water level over a 30- the installation of a septic system. The steps for minute period and calculate the percolation making a soil percolation test (perc test) are as rate. follows: 4. Soils with a percolation rate less than 2.5 centimeters (1 in.) per hour are unsuited for 1. Dig six or more test holes about 15 cen- filter fields because of the danger of satura- ti meters in diameter and to the depth of the tion of the soil above the filter bed with efflu- proposed drainage lines or seepage bed; the ent and the leakage of effluent from the soil FIGURE 6.17 The layout of a septic tank and filter field for disposal of wastewater (effluent) through the soil.
96 SOIL WATER MANAGEMENT surface. Soils with percolation rates greater Land Disposal of Municipal Wastewater than 25 centimeters (10 in.) per hour are un- A relatively small community of 10,000 people suited for filter fields because rapid move- may produce about a million gallons of waste- ment of effluent might pollute groundwater. water, or sewage effluent, per day from its sewage treatment plant. The effluent looks much like ordi- Within the acceptable range of percolation nary tap water; when chlorinated, it is safe forvalues, increasing percolation rates result in de- drinking. Discharging the effluent into a nearbycreasing seepage-bed area. The more people liv- stream does not create a health hazard. The ef-i ng in a dwelling, the more effluent for disposal. A fluent, however, is enriched with nutrients, sograph is used to determine the area of seepage plant growth may be greatly increased in streamsbed needed based on soil percolation rate and and lakes where the effluent is discharged. Weednumber of house occupants. growth in lakes and streams may reduce fish pop- Several other factors are considered in selec- ulations and interfere with boating andtion of filter field or seepage-bed sites. They swimming. To combat these problems, research-should be located at least 50 feet from any stream, ers at the Pennsylvania State University designedopen ditch, or other watercourse into which ef- experiments to use the soil as a living filter tofluent might escape, and the land slope should be remove the nutrients in sewage effluent. The wa-less than 10 percent. Further, septic seepage beds ter was applied with sprinklers, as shown in Fig-should never be installed in poorly drained soils ure 6.18.or where soils become seasonally saturated with As the effluent water percolates through thewater (see Figure 6.5). There is danger that shal- soil, nutrients are absorbed by plant roots andl ow groundwater may be contaminated with dis- micoorganisms. More water is applied than lostease organisms and that water may seep out of the by evapotranspiration and the excess water per-soil surface and contaminate work or play areas. colates from the bottom of the soil and is added to FIGURE 6.18 Application of wastewater (effluent) in a forest with a sprinkler system in winter at Pennsylvania State University. (Photo courtesy USDA.)
PRESCRIPTION ATHLETIC TURF 97the underground water reservoir (aquifer). A com- nomically and environmentally sound. An exam-munity of 10,000 people would need 50 hectares ple of a wastewater land disposal system that uses(about 125 acres) and a weekly application of 5 nutrients for plant growth and return of water to ancentimeters of water to dispose of their effluent. underlying aquifer is shown in Figure 6.19.Under these conditions about 80 percent of theeffluent water will contribute to aquifier recharge PRESCRIPTION ATHLETIC TURFand about 20 percent will be evapotranspired. Inthe Pennsylvania State University experiment, analmost complete removal of the nitrogen and Prescription athletic turf (PAT) is a system of pro-phosphorus in the effluent occurred-the two nu- ducing vigorous turf for use on athletic fields. Ittrients most closely tied to eutrophication. Fur- combines several aspects of water management,thermore, tree and crop growth on the land were i ncluding irrigation and drainage, and was devel-greatly stimulated. oped at Purdue University, where football is Today, more and more cities are utilizing land played on natural grass turf.disposal of sewage effluent. In some areas, the The existing turf area is excavated to a depth oftrees produced in the disposal area are about 18 inches and the soil is removed. A plasticchipped and used as fuel to dry and process the liner is laid on the newly exposed surface, 2-inchsolid sewage sludge. Marketing the sludge as a diameter slitted plastic drain lines are laid on topfertilizer results in a considerable cost saving in of the plastic, and sensors for measuring watersewage disposal and conservation of nutrients. potential and salinity are installed. The mainAdditional benefits include the existence of a pro- features of the PAT system are shown in Figuretected forested or green zone near the community 6.20. The sand layer forms the base for a 2-inch-and recharge of underground water aquifers. Such thick surface layer of sand, peat, and soil. Thewastewater management systems can be eco- water potential sensors maintain a thin layer of FIGURE 6.19 Sewage-treatment system designed to apply wastewater to the land for crop and forest production. The water in excess of evaporation percolates to the water table and contributes to the groundwater (aquifer). (Modified from Woodwell, 1977.)
98 SOIL WATER MANAGEMENT FIGURE 6.20 Diagram of l ayout for prescription athletic turf, PAT. The drainage lines remove excess water in wet season and are used for subirrigation in dry seasons. Sensors monitor water and salinity. (Data provided by Prescription Athletic Turf, Inc.)water above the plastic liner. Grass roots are sup- content of soil, a decrease in the water content,plied by water that moves slowly upward by capil- and a decrease in the matric water potential.larity. During periods of rain, pumps pull excess Irrigation water contains soluble salts andwater out of the soil so that the field is playable some provision (e.g., drainage) must be made towith firm traction regardless of the weather. The maintain a favorable salt balance to keep irrigatedsalt sensors monitor the buildup of salts from soils permanently productive. Salt accumulationirrigation and fertilization. The sandy nature of the results in formation of saline soils. The accumu-soil resists compaction by traffic. l ation of exchangeable sodium, greater than about 15 percent of the adsorbed cations, results i n the formation of sodic soils. High sodium satu- ration results in a deterioration of soil structureSUMMARY and a loss of soil water permeability. Sodic soilsThe three approaches to water management on can be improved by applying gypsum and thenagricultural fields include the conservation of nat- leaching the soil.ural precipitation in arid and subhumid regions, Wastewater (sewage effluent) disposal on thethe removal of water (drainage) from wetlands, l and results in removal of nutrient ions and returnand irrigation to supplement natural precipi- of water to the underlying aquifers.tation. Prescription athletic turf is an artifical system Protection of the land surface by vegetation and for the production of vigorous grass on athleticplant residues shields the soil surface from rain- fields by precise control of water, aeration,drop impact and encourages water infiltration. drainage, salinity, and nutrient supply.Summer fallowing results in storage of water forcrop production at a later time. Fertilizer use mayi ncrease the efficiency of water use by plants byi ncreasing the rate of plant growth. Removal of water by drainage provides an aer- REFERENCESated root zone for crops and dry soil for building Baker, K. F., ed. 1957. "The U. C. System for Producingfoundations. Upward from the surface of a water- Healthy Container-Grown Plants." Ca. Agr. Exp. Sta.saturated soil layer, there is an increase in the air Manual 23. Berkeley.
REFERENCES 99Bender, W. H. 1961. "Soils Suitable for Septic Tank ity of Our Environment. AAAS Pub. 85, Washing- Filter Fields." Agr. Information Bull. 243. USDA, ton, D.C. Washington, D.C. Kemper, W. D., T. J. Trout, A. Segeren, and M. Bullock.Bernstein, L. 1964. "Salt Tolerance of Plants." Agr Infor- 1987. "Worms and Water." J. Soil and Water Conser- mation Bull. 283. USDA, Washington, D.C. vation. 42:401-404.Bernstein, L. 1965. "Salt Tolerance of Fruit Trees." Agr. Mathews, O. R. 1951. "Place of Summer Fallow in the Information Bull. 292. USDA, Washington, D.C. Agriculture of the Western States." USDA Cir. 886.Bower, C. A., 1974. "Salinity of Drainage Waters." Washington, D.C. Drainage in Agriculture. Am. Soc. Agron. Madison. Miller, D. E. and Richard O. Gifford. 1974. "Modification pp. 471-487. of Soil Crusts for Plant Growth," in Arizona Agr. Exp.Edminister, T. W. and R. C. Reeve. 1957. "Drainage Sta. Tech. Bull. 214. Problems and Methods." Soil. USDA Yearbook of Ag- Soil Improvement Comm. Calif. Fert. Assoc. 1985. riculture. Washington, D.C., pp. 379-385. Western Fertilizer Handbook. 7th ed. Interstate, Dan-Evans, C. E. and E. R. Lemon. 1957. "Conserving Soil ville, Ill. Moisture." Soil. USDA Yearbook of Agriculture. Terry, T. A. and J. H. Hughes. 1975. "The Effects of Washington, D.C. Intensive Management on Planted Loblolly PineHanks, R. J. and C. B. Tanner. 1952. "Water Consump- Growth on the Poorly Drained Soils of the Atlantic tion by Plants as Influenced by Soil Fertility. "Agron. J. Coastal Plain." Proc. Fourth North Am. Forest Soils 44:99. Conf. Univ. Laval, Quebec.Jacobsen, T. and R. M. Adams. 1958. "Salt and Silt in United States Salinity Laboratory Staff, 1969. "Diagnosis Ancient Mesopotamian Agriculture." Science. and Improvement of Saline and Alkali Soils." USDA 128:1251-1258. Agr. Handbook 60. Washington, D.C.Kardos, L. T. 1967. "Waste Water Renovation by the Woodwell, G. M. 1977. "Recycling Sewage through Land-A Living Filter." in Agriculture and the Qual- Plant Communities." Am. Scientist 65:556-562.
CHAPTER 7 SOIL EROSIONSoil formation and soil erosion are two natural banks by water causes large masses of soil to falland opposing processes. Many natural, undis- i nto the stream and be carried away. Very wet soilturbed soils have a rate of formation that is bal- masses on steep slopes are likely to slide slowlyanced by a rate of erosion. Under these condi- downhill (solifluction).tions, the soil appears to remain in a constant Raindrops falling on bare soil detach particlesstate as the landscape evolves. Generally, the and splash them up into the air. When there isrates of soil erosion are low unless the soil surface little water at the soil surface, the water tends tois exposed directly to the wind and rainwater. The run over the soil surface as a thin sheet, causingerosion problem arises when the natural vegeta- sheet erosion. Rill and gully erosion are due to thetive cover is removed and rates of soil erosion are energy of water that has concentrated and is mov-greatly accelerated. Then, the rate of soil erosion i ng downslope. As water concentrates, it firstgreatly exceeds the rate of soil formation and forms small channels-called rills-followed bythere is a need for erosion control practices that greater concentrations of water. Large concentra-will reduce the erosion rate and maintain soil tions of water lead to gully formation. Some of theproductivity. highest soil erosion rates occur at construction Erosion is a three-step process: detachment fol- sites where the vegetation has been removed andlowed by transport and deposition. The energy for the soil is exposed to the erosive effects of rain, aserosion is derived from falling rain and the sub- shown in Figure 7.1. The tendency of water tosequent movement of runoff water or the wind. collect into small rills that coalesce to form large channels and produce gully erosion is also shown in Figure 7.1.WATER EROSION Predicting Water Erosion Rates onSeveral types of water erosion have been iden- Agricultural Landtified: raindrop (splash), sheet, rill, and gully or The rate of soil erosion on agricultural land ischannel. In addition, undercutting of stream affected by rainfall characteristics, soil erodibility, 100
WATER EROSION 101 These quantative data form the basis for pre- dicting erosion rates by using the Universal Soil- Loss Equation (USLE) developed by Wischmeier and Smith. The USLE equation is where A is the computed soil loss per unit area as tons per acre, R is the rainfall factor, K is the soil-erodibility factor, L is the slope-length factor, S is the slope-gradient factor, C is the cropping- management factor, and P is the erosion-control practice factor. The equation is designed to pre- dict water erosion rates on agricultural land sur-FIGURE 7.1 faces, exclusive of erosion resulting from the for- Removal of the vegetative cover mation of large gullies.i ncreases water runoff and leaves soil unprotectedfrom the erosive effects of the rain, resulting in R = The Rainfall Factoraccelerated soil erosion. As water concentrates inchannels, its erosive power increases. (Photographcourtesy Soil Conservation Service, USDA.) The rainfall factor (R) is a measure of the erosive force of specific rainfall. The erosive force, or available energy, is related to both the quantityslope characteristics, and vegetative cover and/or and intensity of rainfall. A 5-centimeter rain fallingmanagement practices. The first plots in the at 32 kilometers per hour (20 mph) would have 6United States for measuring runoff and erosion, as million foot-pounds of kinetic energy. The tre-i nfluenced by different crops, were established at mendous erosive power of such a rain is sufficientthe University of Missouri in 1917 (see Figure 7.2). to raise an 18-centimeter-thick furrow slice to aSince 1930, many controlled studies on field plots height of 1 meter. The four most intense storms inand small watersheds have been conducted to a 10-year period at Clarinda, Iowa, accounted forstudy factors affecting erosion. Experiments were 40 percent of the erosion and only 3 percent of theconducted on various soils at many different loca- water runoff on plots planted with corn that weretions in the United States, using the same stan- tilled up and down the slope. Anything thatdard conditions. For example, many erosion plots protects the soil from raindrop impact, such aswere 72.6 feet long on 9 percent slopes and were plant cover or stones, protects the soil from ero- subjected to the same soil management practices. sion, as shown in Figure 7.3. FIGURE 7.2 Site of the first plots in the United States for measuring runoff and erosion as influenced by different crops. The plots were established in 1917 at the University of Missouri. M. Woodruff, University of (Photograph courtesy Dr. C. Missouri.)
102 SOIL EROSION The rainfall factor (R) is the product of the total kinetic energy of the storms times the maximum 30-minute intensity of fall and modified by any influence of snowmelt. Rainfall factors have been computed for many locations, with values ranging from less than 20 in western United States to 550 along the Gulf Coast of the southeastern United States. An iso-erodent map of average annual values of R is shown in Figure 7.4. K = The Soil Erodibility Factor Soil factors that influence erodibility by water are:FIGURE 7.3 Columns of soil capped by stones that (1) those factors that affect infiltration rate, per-absorbed the energy of raindrop impact and meability, and total water retention capacity, andprotected the underlying soil from erosion. (2) those factors that allow soil to resist disper-
WATER EROSION 103sion, splashing, abrasion, and the transporting point where the slope decreases to the extent thatforces of rainfall and runoff. The erodibility factor deposition occurs, or a point where water runoff(K) has been determined experimentally for 23 enters a well-defined channel. Runoff from themajor soils on which soil erosion studies were upper part of a slope contributes to the totalconducted since 1930. The soil loss from a plot amount of runoff that occurs on the lower part of72.6 feet long on a 9 percent slope maintained in the slope. This increases the quantity of waterfallow (bare soil surface and with no crop running over the lower part of the slope, thusplanted) and with all tillage up and down the i ncreasing erosion more on the lower part of theslope, is determined and divided by the rainfall slope than on the upper part. Studies have shownfactor for the storms producing the erosion. This that erosion via water increases as the 0.5 poweris the erodibility factor. Values of K for 23 soils are of the slope length (L ° 5). This is used to calculatelisted in Table 7.1. the slope length factor, L. This results in about 1.3 ti mes greater average soil loss per acre for every The Slope Length and Slope doubling of slope length.LS = As the slope gradient (percent of slope) in-Gradient Factors creases (S), the velocity of runoff water increases,Slope length is defined as the distance from the which in turn increases the waters erosive power.point of origin of overland water flow either to a A doubling of runoff water velocity increases ero- TABLE 7.1 Computed K Values for Soils From Wischmeier and Smith, 1965. a Evaluated from continuous fallow. All others were computed from row crop data.
104 SOIL EROSIONsive power four times, and causes a 32-time in- all interrelated crop cover and management vari-crease in the amount of material of a given parti- ables including type of tillage, residue manage-cle size that can be carried in the runoff water. ment, and time of soil protection by vegetation.Splash erosion by raindrop impact causes a net Frequently, different crops are grown in sequencedownslope movement of soil. Downward slope or in rotation; for example, corn is grown one yearmovement also increases with an increase in the followed by wheat, which is then followed by aslope gradient. Combined slope length and gradi- hay or meadow crop. This would be a three-yearent factors (LS) for use in the soil-loss prediction rotation of corn-wheat-meadow (C-W-M). Theequation are given in Figure 7.5. benefit of a rotation is that a year of high erosion, during the corn production year, is averaged withC = The Cropping-Management Factor years of lower erosion to give a lower average erosion rate, than if corn was planted every year.A vegetative cover can absorb the kinetic energy However, when corn is grown no-till, there mayof falling raindrops and diffuse the rains erosive be little soil erosion because of the protection ofpotential. Furthermore, vegetation by itself retains the soil by previous crop residues, as shown in a significant amount of the rain and slows the flow Figure 7.7. of runoff water. As a result, the presence or ab- Complicated tables have been devised for cal- sence of a vegetative cover determines whether culating the C factor. As an illustration, the C erosion will be a serious problem. This is shown factor for a 4-year rotation of wheat-alfalfa and in Figure 7.6 where erosion on plots 72.6 feet long bromegrass-corn-corn (W-AB-C-C) in central In- with an 8 percent slope had about 1,000 times diana with conventional tillage, average residue more erosion when in continuous corn as com- management, and average yields is 0.119. Some pared with continuous bluegrass. comparison values are as low as 0.03 for corn The C factor measures the combined effect of mulched in no-till with 90 percent of the soil Slope length and gradient graph for determining the FIGURE 7.5 topographic factor, LS.
WATER EROSION 105 fered by sod or closely growing crops in the sys- tem needs to be supported by practices that will slow runoff water, thus reducing the amount of soil carried. The most important of these prac- tices for croplands are contour tillage, strip cropping on the contour, and terrace systems. Contour tillage is tillage operations and planting on the contour as contrasted to tillage up and down the slope or straight across the slope. No-till planting of corn on the contour between terraces i n an Iowa field is shown in Figure 7.8. Limited field studies have shown that contour farming alone is effective in controlling erosion during rainstorms of low or moderate intensity, but it provides little protection against the occa- sional severe storm that causes breakovers byFIGURE 7.6 A continuous plant cover (e.g., runoff waters of the contoured ridges or rows. Pbluegrass) is many times more effective for erosion values for contouring are given in Table 7.2, fromcontrol than cropping systems that leave the land which it is apparent that contouring is the mostbare most of the time. Continuous corn production effective on slopes of 2.1 to 7 percent. Stripresulted in the longest amount of time with the soil cropping is the practice of growing two or moreunprotected by vegetation and the most erosion. crops in alternating strips along contours, or per-covered with crop residues and 0.48 for continu- pendicular to surface water flow. Strip cropping,ous corn with conventional tillage. together with contour tillage, provides moreP = The Erosion Control Practice Factor protection. In cases where both strip cropping and contour tillage are used, the P values listed inI n general, whenever sloping land is cultivated Table 7.2 are divided by 2.and exposed to erosive rains, the protection of- Terraces are ditches that run perpendicular to FIGURE 7.7 No-till corn with the residues of a previous soybean crop providing protection against soil erosion. (Photograph courtesy Soil Conservation Service, USDA.)
106 SOIL EROSION FIGURE 7.8 Factors contributing to soil erosion control in this field include no-till tillage on the contour with terraces that greatly reduce the effective length of slope. (Photograph courtesy Soil Conservation Service, USDA.)the direction of surface water flow and collect ceptable levels of erosion. Having been devel-runoff water. The grade of the channel depends oped under the conditions found in the Unitedon the amount of runoff water. In areas of limited States, one must be cautious in applying the equa-rainfall and permeable soil, channels may have a tion to drastically different conditions, such aszero grade. In humid regions, the terrace channel exist in the tropics. The procedure for computingusually is graded, and the runoff water that is the expected, average annual soil loss from acollected by the terrace is carried to an outlet for particular field is illustrated by the following ex-disposal. Terraces are an effective way to reduce ample.slope length. To account for terracing, the slope Assume that there is a field in Fountain County,length used to determine the LS factor should Indiana, on Russell silt loam. The slope has arepresent the distance between terraces. Short gradient of 8 percent and is about 200 feet long.slope length between terraces is shown in Fig- The cropping system is a 4-year rotation of wheat-ure 7.8. alfalfa and bromegrass-corn-corn (W-AB-C-C) with tillage and planting on the contour, and with corn residues disked into the soil for wheat seed-Application of the Universal i ng and turned under in the spring for second-yearSoil-Loss Equation corn. Fertility and residue management on thisThe USLE was designed to predict the long-term farm are such that crop yields per acre are abouterosion rates on agricultural land so that manage- 100 bushels of corn, 40 bushels of wheat, and 4ment systems could be devised that result in ac- tons of alfalfa-bromegrass hay. The probability of an alfalfa-bromegrass failure is slight, and there is little risk that serious erosion will occur during theTABLE 7.2 Practice Factor Values for Contouring alfalfa-bromegrass year due to a lack of a good vegetative cover. The first step is to refer to the charts and tables discussed in the preceding sections and to select the values of R, K, LS, C, and P that apply to the specific conditions on this particular field. The value of the rainfall factor (R) is taken from Figure 7.4. Fountain County, in west-central In-From Wischmeier and Smith, 1965. diana, lies between isoerodents of 175 and 200.
WATER EROSION 107By linear interpolation, R equals 185. Soil scien- emphasis is on the maintenance of a certain thick-tists consider that the soil erodibility factor (K) for ness of soil overlying any impermable layer thatRussell silt loam is similar to that for Fayette silt might exist.l oam, being 0.38, according to Table 7.1. Second, the T value is the maximum average The slope-effect chart, Figure 7.5, shows that, annual soil loss that will allow continuousfor an 8 percent slope, 200 feet long, the LS is 1.41. cropping and maintain soil productivity withoutFor the productivity level and management prac- requiring additional management inputs. Fertil-tices, assumed in this example, the factor C for a izers are used to overcome the decline in soilW-AB-C-C rotation was shown earlier to be 0.119. fertility caused by erosion.Table 7.2 shows a practice factor of 0.6 for con- On thick, water-permeable soils, such astouring on an 8 percent slope. Marshall silt loam in western Iowa, artificial re- The next step is to substitute the selected nu- moval of the surface soil reduced yields to less merical values for the symbols in the soil erosion than one half of the crop yield obtained from equation, and solve for A. In this example: uneroded soil when no fertilizer was applied. The application of fertilizer containing 200 pounds of nitrogen per acre, however, resulted in similar yields of corn on both the normal soil and the soil If planting had been up and downslope instead with the surface soil removed. The Marshall soil isof on the contour, the P factor would have been a thick permeable soil that developed in a thick1.0, and the predicted soil loss would have been l ayer of loess (parent material). Loess is a mate-185 x 0.38 x 1.41 x 0.119 x 1.0 = 11.8 tons per rial transported and deposited by wind and con-acre (26.4 metric tons per hectare). sisting predominantly of silt-sized particles. Many If farming had been on the contour and com- soils that have developed from thick sediments ofbined with minimum tillage for all corn in the l oess are agriculturally productive.rotation, the factor C would have been 0.075. The There was some evidence that in years of mois-predicted soil loss would then have been 4.5 tons ture stress, the normal fertilized Marshall silt loamper acre (10.1 metric tons per hectare). outproduced the soil where the surface soil had been removed and fertilized. As a result of this and similar experiments, another approach wasThe Soil Loss Tolerance Value developed to establish Tvalues. This method con-The soil loss tolerance value (T) has been defined siders the maximum soil loss consistent with thei n two ways. This is an indication of the lack of maintenance of productivity in terms of the main-concensus among soil scientists as to what ap- tenance of a rooting depth with desirable physi-proach should be taken with T values or how cal properties. Where subsoils have physicalmuch erosion should be tolerated. First, the T properties unsuitable for rooting, erosion resultsvalue is the maximum soil erosion loss that is i n reductions in soil productivity that cannot beoffset by the theoretical maximum rate of soil overcome with only fertilizer application. Thedevelopment, which will maintain an equilibrium data in Figure 7.9 show that removing the surfacebetween soil losses and gains. Shallow soils over l ayer from a fine-textured Austin soil in the Black-hard bedrock have small T values. Erosion on l ands of Texas resulted in longtime or permanentsoils with claypans will result in shallower rooting reductions in soil productivity. Soil loss tolerancedepths and in the need for smaller T values than value guidelines from the Soil Conservation Ser-for thick permeable soils, when both kinds of vice of the USDA are given in Table 7.3.soils have developed from thick and water- The data in Figure 7.6 show that average annualpermeable unconsolidated parent materials. The soil loss with continuous bluegrass was 0.03 tons
108 SOIL EROSION suited in a soil loss exceeding 50 tons per acre or 2.5 centimeters (1 inch) every 2 to 3 years, a loss far exceeding the Tvalue. In the Indiana case, the calculated soil losses ranged from about 4 to 12 tons per acre. The average annual rate of soil erosion on cropland land in the United States is 5 tons per acre. It has been estimated that soil ero- sion is the most important conservation problem on 51 percent of the nations cropland. In general, Tvalues in the United States range from 1 to 5 tons per acre (2 to 11 metric tons per hectare). The USLE can be used to determine the management required to maintain soil losses at or below a certain T value. Water Erosion on Urban Lands Historic soil erosion rates have changed as land use has changed. Much of the land in the easternFIGURE 7.9 United States, which contributed much sediment Effect of removal of 38 centimeters oftopsoil on grain sorghum yields on Austin clay at to streams and rivers a hundred or more yearsTemple, Texas, 1961-1965. (Data from Smith et al., ago, has been removed from cropping. Lower ero-1967.) sion rates resulted from abandonment of land for cropping and returning it to forest. In the past fewper acre and resulted in the removal of about 2.5 decades, much of this land has experienced acentimeters (1 in.) of soil every 5,000 years. Obvi- high erosion rate because of urbanization, con-ously, this soil loss is well below the T value for struction of shopping centers, roads, and housingthe soil. On the other hand, continuous corn re- areas, as shown in Figure 7.10. TABLE 7.3 Guide for Assigning Soil Loss Tolerance Values to Soils with Different Rooting Depths Data from USDA-SCS, 1973. ° t Soils that have a favorable substratum and can be renewed by tillage, fertilizer, organic matter, and other management practices. $ Soils that have an unfavorable substratum, such as rock, and cannot be renewed economically.
WATER EROSION 109 Now, about 50 percent of the sediment in where erosion occurs. Excessive water runoff andstreams and waters throughout the country origi- soil erosion contribute to flooding and addition ofnates on agricultural land; the other 50 percent fresh sediments to the soils of the north-Chinaoriginates on urban lands. It has been noted that plain.exposed land on steep slopes is very susceptible Soil erosion results in the preferential loss ofto erosion. In fact, exposure of land during high- the finer soil components that contribute the mostway and building construction commonly results to soil fertility. Loss of organic matter, and thei n erosion rates many times those that typically subsequent reduction in nitrogen mineralization,occur on agricultural land. Urban erosion rates as are particularly detrimental to the production of l arge as 350 tons per acre in a year have been nonleguminous grain crops. As mentioned, thisreported. It is important to plan construction so l oss can in some cases, be overcome by the addi-that land will be exposed a minmum period of tion of nitrogen fertilizer on responsive soils.ti me (see Figure 7.11). However, there is an increase in the cost of crop production. Exposure of clay-enriched Bt horizons (be-Water Erosion Costs cause of surface soil erosion) reduces water in-I n northern China, there is an extensive loess de- filtration and increases water runoff. This resultsposit that is locally over 80 meters thick. This i n more soil erosion and retention of less water forl oess area is the major source of sediment for the plant growth. Seedbed preparation is more diffi-Huang Ho (Yellow) River. This sediment is the cult and the emergence of small plant seedlingsmajor parent material for the alluvial soils of the may be reduced, resulting in a less than desirablevast north-China plain, which supports millions of density of plants. Summarization of data frompeople. The loessial landscape is very distinctive, many studies showed that 2.5 centimeters (1 in.)with deeply entrenched streams and gullies. It has of soil loss reduced wheat yield 5.3 percent, cornbeen estimated that the loess will be removed by 6.3 percent, and grain sorghum 5.7 percent. Some erosion in 40,000 years, which would be an an- effects of soil erosion on crop growth and produc- nual soil loss of 15 tons per acre (33 metric tons tion costs are given in Table 7.4. Increased pro- per hectare). This appears to be an excessive soil duction costs justify a long-term investment in erosion rate. erosion control measures to maintain erosion at The costs and benefits of the erosion and sedi- or below acceptable T values. mentation are difficult to assess. In the short term, Off-site costs result from sediment in water set- food production costs are increased in areas tling and filling reservoirs and navigation chan-
11 0 SOIL EROSION FIGURE 7.11 Slurry truck spraying fertilizer and seed on freshly prepared seedbed along road in Missouri. A thin l ayer of straw mulch will be secured with a thin spray of tar to protect the soil until vegetation becomes established. (Photograph courtesy Soil Conservation Service, USDA.)nels. The sediments adversely affect the ecology i n diameter) are rolled over the surface by directof streams and lakes, as well as municipal water wind pressure, then suddenly jump up almostsupplies. In 1985 it was estimated that the cost of vertically over a short distance to a height of 20 tosoil erosion resulting from all phases of pollution, 30 centimeters. Once in the air, particles gain inboth agricultural and urban, was $6 billion annu- velocity and then descend in an almost straightally in the United States. This is a loss greater than line, not varying more than 5 to 12 degrees fromthat estimated for the loss in soil productivity from the horizontal. The horizontal distance traveled bythe eroded land. a particle is four to five times the height of its j ump. On striking the surface, the particles may rebound into the air or knock other particles intoWIND EROSION the air before coming to rest. The major part of soil carried by wind moves by this process. AboutWind erosion is indirectly related to water conser- 93 percent of the total soil movement via windvation in that a lack of water leaves land barren takes place below a height of 30 centimeters.and exposed to the wind. Wind erosion is greatest Probably 50 percent or more of movement occursin semiarid and arid regions. In North Dakota, between 0 and 5 centimeters above the soilwind-erosion damage on agricultural land was surface.reported as early as 1888. In Oklahoma, soil ero- Very fine dust particles are protected from windsion was reported 4 years after breaking of the action, because they are too small to protrudesod. Blowing dust was a serious hazard of early above a minute viscous layer of air that clings tosettlers on the Plains, and was one of the first the soil surface. As a result, a soil composed ofproblems studied by the Agricultural Experiment extremely fine particles is resistant to wind ero-Stations of the Great Plains. sion. These dust-sized particles are thrown into the air chiefly by the impact of particles moving inTypes of Wind Erosion saltation. Once in the air, however, their move- ment is governed by wind action. They may beSoil particles move in three ways during wind carried very high and over long distances via sus-erosion: saltation, suspension, and surface creep. pension.During saltation, fine soil particles (0.1 to 0.5 mm Relatively large sand particles (between 0.5 and
WIND EROSION 11 1 TABLE 7.4 Effects of Erosion on Corn Growth and Production Costs1.0 mm in diameter) are too heavy to be lifted by field length, and V equals the quantity of vegeta-wind action, but they are rolled or pushed along tive cover. Wind erosion tolerance levels (Tthe surface by the impact of particles during salta- values) have been established, just as for watertion. Their movement is by surface creep. erosion. The use of the wind erosion equation Between 50 and 75 percent of the soil is carried is more complex than that used for calculatingby saltation, 3 to 40 percent by suspension, and 5 water erosion losses. Details for use of theto 25 percent by surface creep. From these facts, it wind erosion equation are given in the Nationalis evident that wind erosion is due principally to Agronomy Manual of the Soil Conservation Ser-the effect of wind on particles of a size suitable to vice, 1988.move in saltation. Accordingly, wind erosion can Factors Affecting Wind Erosionbe controlled if: (1) soil particles can be built upi nto peds or granules too large in size to move bysaltation; (2) the wind velocity near the soil sur- Soil erodibility by wind is related primarily to soilface is reduced by ridging the land, by vegetative texture and structure. As the clay content of soilscover, or even by developing a cloddy surface; i ncreases, increased aggregation creates clods orand (3) strips of stubble or other vegetative cover peds too large to be transported. A study in west-are sufficient to catch and hold particles moving ern Texas showed that soils with 10 percent clayi n saltation. eroded 30 to 40 times faster than soil with 25 percent clay, as shown in Figure 7.12.Wind Erosion Equation Rough soil surfaces reduce wind erosion by reducing the surface wind velocity and by trap-The factors that affect wind erosion are contained ping soil particles. Surface roughness can be in-i n the equation to estimate soil loss by wind creased with tillage operations, as shown in Fig-erosion: ure 7.13. Tillage and rows are positioned at right angles to the dominant wind direction for maxi- mum effectiveness.where: E equals the soil loss in tons per acre, I Climate is a factor in wind erosion via its effectequals the soil erodibility, K equals the soil rough- on wind frequency and the velocity and wetnessness, C equals the climatic factor, L equals the of soil during high-wind periods. Wind erosion
11 2 SOIL EROSION and blowing dust are persistent features of deserts i n which plants are widely spaced and dry soil is exposed most of time. When surface soils contain a wide range in particle sizes, wind erosion preferentially removes the finer particles, leaving the larger particles behind. In this way a desert pavement is created, consisting of gravel particles that are too large and heavy to be blown away. Consequently, deserts are an important source of loess, which is composed mainly of silt-sized particles. Furthermore, climate affects the amount of vegetative cover. During the Dust Bowl of the 1930s, both wheat yields and wind erosion were related to rainfall. The lowest yields and highest soil erosion rates occurred during years of least rainfall. Wind erosion increases from zero, at the edge of an open field, to a maximum, with increasing distance of soil exposed to the wind. As particlesFIGURE 7.12 Amount of wind erosion in relation to bounce and skip, they create an avalanche effectthe amount of clay in the soil. (Data from Chepil et on soil movement. Windbreaks of trees and al-al., 1955.) ternate strips of crops can be used to reduce the FIGURE 7.13 Contour ridges reduce wind velocity and collect much of the blowing soil. (Photograph courtesy soil Conservation Service, USDA.)
REFERENCES 11 3effective field length exposed to the wind. Finally, with a heavy roller induces moisture to rise morewind erosion is inversely related to the degree of rapidly by capillarity, thus dampening the soilvegetative cover. Growing crops and the residues surface. The water table must be quite close to theof previous crops are effective in controlling wind soil surface for this practice to be effective.erosion. Stubble mulch tillage-tillage that leaves SUMMARYplant residues on the soil surface-is popular inthe wheat producing region of the Great Plains. Deep Plowing for Wind Erosion Control The factors that affect soil erosion by water are rainfall, soil erodibility, slope length and gradient,Deep plowing has been used to control wind ero- crop management practices, and erosion controlsion where sandy surface soils are underlain by Bt practices. The universal soil loss equation is usedhorizons that contain 20 to 40 percent clay. In to estimate soil erosion loss on agricultural land. Kansas and Texas, plowing to control wind ero- The permissible soil loss is the T value. Thesion must be deep enough to include at least I universal soil loss equation can be used to deter-centimeter of Bt horizon for every 2 centimeters of mine the soil management systems that will resultthe surface soil (A horizon) thickness. In many i n acceptable erosion levels.places, the soil must be plowed to a depth of 50 to Soil erosion is greatly accelerated when the60 centimeters. This kind of plowing increases the vegetative cover is removed. Thus, soil erosion isclay content of the surface soil by 5 to 12 percent, a serious problem in both rural and urban areas.i n some cases. Since 27 percent clay in the sur- Cost of soil erosion includes reduced soil pro-face was required to halt soil blowing, deep plow- ductivity and higher crop production costs. Offsitei ng by itself resulted in only partial and temporary costs are due to silting of watercourses and reser-control of wind erosion. When deep plowing was voirs and reduced water quality.not accompanied by other soil erosion control Wind erosion is a major problem when sandypractices, wind erosion removed clay from the soils are used for crop production in regions withsurface soil, and the effects of deep plowing were dry seasons, where the soil surface tends to betemporary. exposed to the wind because of limited vegetativeWind Erosion Control on Organic Soils cover. Wind erosion control is based on one or more of the following:Organic soil areas of appreciable size are fre-quently protected from wind erosion by planting 1. Trapping soil particles with rough surfacetrees in rows-windbreaks. The use of trees as and/or the use of crop residues and stripwindbreaks must be limited, however, because of cropping.the large amount of soil that they take out of crop 2. Deep plowing to increase clay content of sur-production. A number of shrubs are also used; for face soil and simultaneous use of other ero-example, spirea is popular as a windbreak. Trees sion control practices.and shrubs also provide cover for wildlife. 3. Protecting the soil surface with a vegetative The lightness of dry organic soils contributes to cover.the wind erosion hazard. Moist organic soil is REFERENCESheavier than dry organic soils, and particles tendto adhere more, reducing the wind erosion haz-ard. Accordingly, use of an overhead sprinkler Anonymous. 1982. Soil Erosion: Its Agricultural, Envi-irrigation system is helpful during windy periods, ronmental, and Socioeconomic Implications. Coun-before the crop cover is sufficient to provide cil for Agr. Sci. and Technology Report No. 92, Ames,protection from the wind. Rolling the muck soil Iowa.
11 4 SOIL EROSIONBeasley, R. P. 1974. "How Much Does Erosion Cost?" and D. 0. Thompson. 1967. "Renewals of Desurfaced Soil Survey Horizons. 15:8-9. Austin Clay." Soil Sci. 103:126-130.Browning, G. M., R. A. Norton. A. G. McCall, and F. G. Soil Conservation Service, 1988. National Agronomy Bell. 1948. "Investigation in Erosion Control and the Manual. USDA, Washington, D.C. Reclamation of Eroded Land at the Missouri Valley Tuan, Yi-Fu. 1969. China. Aldine, Chicago. Loess Conservation Experiment Station." USDA Tech USDA-SCS. 1973. Advisory Notice, 6. Washington, D.C. Bull. 959, Washington, D.C. Wischmeier, W. H. and D. D. Smith. "PredictingBurnett, E., B. A. Stewart, and A. L. Black. 1985. "Re- Rainfall-Erosion Losses from Cropping East of the gional Effects of Soil Erosion on Crop Productivity- Rocky Mountains." USDA Agr. Handbook 282. Wash- Great Plains." Soil Erosion and Crop Productivity. i ngton, D.C. American Society of Agronomy, Madison, Wis. Wischmeier, W. H. and D. D. Smith. 1978. "PredictingChepil, W. S., N. P. Woodruff, and A. W. Zingg. 1955. Rainfall Erosion Loss-A Guide to Conservation Plan- "Field Study of Wind Erosion in Western Texas." ning." USDA Agr. Handbook 537. Washington, D.C. Kansas and Texas Agr. Exp. Sta. and USDA Washing- Woodruff, N. P. and F. H. Siddoway. 1965. "A Wind ton, D.C. Erosion Equation." Soil Sci. Soc. Am. Proc. 29:602-Colacicco, D., T. Osborn, and K. Alt. 1989. "Economic 608. Damage from Soil Erosion." J. Soil and Water Con. Wolman, M. G. 1967. "A Cycle of Sedimentation and 44:(1), 35-39. Erosion in the Urban River Channels." Geog. Ann.Massey, H. F. and M. L. Jackson. 1952. "Selective Ero- 49A:385-395. sion of Soil Fertility Constitutents." Soil Sci. Soc. Am. Zobeck, T. M., D. W. Fryrear, and R. D. Petit. 1989. Proc. 16:353-356. "Management Effects on Wind-eroded Sediment andSmith, R. M., R. C. Henderson, E. D. Cook, J. E. Adams, Plant Nutrients." J Soil Water Con. 44:160-163.
CHAPTER 8 Im SOIL ECOLOGYThe soil is the home of innumerable forms of THE ECOSYSTEMplant, animal, and microbial life. Some of the The sum total of life on earth, together with thefascination and mystery of this underworld has global environments, constitutes the ecosphere.been described by Peter Farb: The ecosphere, in turn, is composed of numerousWe live on the rooftops of a hidden world. Be- self-sustaining communities of organisms andneath the soil surface lies a land of fascination, their inorganic environments and resources,and also of mysteries, for much of mans wonder called ecosystems. Each ecosystem has its ownabout life itself has been connected with the soil. unique combination of living organisms andIt is populated by strange creatures who have abiotic resources that function to maintain a con-found ways to survive in a world without sunlight, All ecosystems have two types of organisms tinuous flow of energy and nutrients.an empire whose boundaries are fixed by earthenwalls. based on carbon source: (1) producers, and (2) the consumers and decomposers. The producers Life in the soil is amazingly diverse, ranging use (fix) inorganic carbon from carbon dioxide,from microscopic single-celled organisms to and are autotrophs. The consumers and the de-l arge burrowing animals. As is true with organ- composers use the carbon fixed by the producers,isms above the ground, there are well-defined such as glucose, and are heterotrophs.food chains and competition for survival. Thestudy of the relationships of these organisms inthe soil environment is called soil ecology. Producers The major primary producers are vascular plants that use solar energy to fix carbon from carbon* Peter Farb, Living Earth, Harper and Brothers Publishers, dioxide during photosynthesis. The tops of plantsNew York, 1959. provide food for animals above the soil- 115
11 6 SOIL ECOLOGYatmosphere interface. Plants produce roots, tu- of an element from an organic form to an inor-bers, and other underground organs within the ganic state as a result of microbial activity. Ansoil that serve as food for soil-dwelling organisms. example is the conversion of nitrogen in proteinA very small amount of carbon is fixed from car- into nitrogen in ammonia (NH 3). The microor-bon dioxide by algae during photosynthesis that ganisms are considered to be the major or ulti-occurs at or near the soil surface. Some bacteria mate decomposers. They are the most closelyobtain their energy from chemical reactions, che- associated with the ultimate release of the energymoautotrophs, and fix a tiny amount of carbon and nutrients in decomposing materials and infrom carbon dioxide. The material produced by making the nutrients available for another cycle ofthe producers serves as food for the consumers plant growth. Only about 5 percent of the primaryand decomposers. production of green plants is consumed by animals. By contrast, about 95 percent of the pri- mary production is ultimately decomposed by mi-Consumers and Decomposers croorganisms.Consumers are, typically, animals that feed on General Features of Decomposersplant material or on other animals. For example,very small worms invade and eat living roots. Theworms might be eaten or consumed by mites The decomposers secrete enzymes that digest or-which, in turn might be consumed by centipedes. ganic matter outside the cell, and they absorb theEventually, all organisms die and are added to the soluble end products of digestion. Different en-soil, together with the waste material of animals. zymes are excreted, depending on the decom- All forms of dead organic materials are attacked posing material. For example, the enzyme cellu-by the decomposers, mainly by bacteria and lase is excreted to decompose cellulose and thefungi. Through enzymatic digestion (decom- enzyme protease is excreted to decomposeposition), the carbon is returned to the atmo- protein. These enzymes and processes are similarsphere as C0 2 and energy is released as heat. The to those that occur in the digestive system ofnutrients in organic matter are mineralized and animals.appear as the original ions that were previously The microorganisms near plant roots share theabsorbed by plant roots. The result is that the same environment with roots, and they competemajor function of the consumers and decom- with each other for the available growth factors.posers is the cycling of nutrients and energy. In When temperature, nutrients and water supply,this context it is appropriate to consider soil as the pH, and other factors are favorable for plantstomach of the earth. Without decomposers to growth, the environment is generally favorable forrelease fixed carbon, the atmosphere would be- the growth of microorganisms. Both absorb nutri-come depleted of C0 2 , life would cease, and the ent ions and water from the same soil solution.cycle would stop. Both are affected by the same water potentials, osmotic effects of salts, pH, and gases in the soil atmosphere. Both may be attacked by the sameMICROBIAL DECOMPOSERS predators. On the other hand, plants and microor- ganisms also play roles that benefit each other.All living heterotrophs, animals and microor- Roots slough off cells and leak organic com-ganisms, are both consumers and decomposers. pounds that serve as food for microorganisms,They are consumers in the sense that they con- whereas the microbes decompose the organicsume materials for growth and, at the same time, materials, which results in the mineralization ofthey are decomposers in the sense that carbon is nutrient ions for root absorption.released as C02 . Mineralization is the conversion In the case of symbiotic nitrogen fixation, the
MICROBIAL DECOMPOSERS 11 7nitrogen-fixing organisms obtain energy and foodfrom the plant while the plant benefits from thenitrogen fixed. Nitrogen fixation is the conversionof molecular nitrogen (N 2) to ammonia and sub-sequently to organic combinations or to formsutilizable in biological processes. The net effectof the fixation is the transfer of nitrogen in theatmosphere, which is unavailable to plants, toplants and soils where the nitrogen is available. The major distinguishing feature of microor-ganisms is their relatively simple biological orga-nization. Many are unicellular, and even multi-cellular organisms lack differentiation into celltypes and tissues characteristic of plants andanimals. They form spores, or a resting stage,under unfavorable conditions and germinate andgrow again when conditions become favorable.BacteriaBacteria are single celled, among the smallest FIGURE 8.1 Rod-shaped bacteria magnified 20,000living organisms, and exceed all other soil organ- times. (Courtesy Dr. S. Flegler of Michigan Stateisms in kinds and numbers. A gram of fertile soil University).commonly contains 10 to 10 10 bacteria. The mostcommon soil bacteria are rod-shaped, a micron(1 /25,000 of an inch) or less in diameter, and up ply very rapidly under favorable conditions. If ato a few microns long (see Figure 8.1). Research- single bacterium divided every hour, and everyers have estimated that the live weight of bacteria subsequent bacterium did the same, 17 millionin soils may exceed 2,000 kilograms per hectare cells would be produced in a day. Such a rapid(2,000 pounds per acre). The estimated weights growth rate cannot be maintained for a long pe-and numbers of some soil organisms are given in riod of time because nutrients and other growthTable 8.1. factors are exhausted and waste products accu- Most of the soil bacteria are heterotrophs and mulate and become toxic. In general, the growthrequire preformed carbon (such as carbon in or- of the decomposers is chronically limited by theganic compounds). Most of the bacteria are small amount of readily decomposable substrateaerobes and require a supply of oxygen (0 2) in the that is available as a food source. Most of thesoil atmosphere. Some bacteria are anaerobic organic matter in soils is humus; organic materialand can thrive only with an absence of oxygen in that is very resistant to further enzymatic decom-the soil atmosphere. Some bacteria, however, are position.facultative aerobes. They thrive well in an aerobicenvironment but they can adapt to an anaerobicenvironment. Fungi Bacteria normally reproduce by binary fission Fungi are heterotrophs that vary greatly in size and (the splitting of a body into two parts). Some structure. Fungi typically grow or germinate fromdivide as often as every 20 minutes and may multi- spores and form a threadlike structure, called the
11 8 SOIL ECOLOGYmycelium. Whereas the activity of bacteria is lim- Some molds grow on plant leaves and produceited to surface erosion in place, fungi readily ex- white cotton-colored colonies that produce a dis-tend their tissue and penetrate into the surround- ease called downy mildew. Mushroom fungi havei ng environment. The mycelium is the working an underground mycelium that absorbs nutrientsstructure that absorbs nutrients, continues to and water, and an above-ground mushroom thatgrow, and eventually produces reproductive contains reproductive spores. Many mushroomsstructures that contain spores, as shown in Fig- are collected for food, such as the shaggy-maneure 8.2. mushroom shown in Figure 8.3. It is difficult to make an accurate determination Fungi are important in all soils. Their toleranceof the number of fungi per gram of soil, because for acidity makes them particularly important in mycelia are easily fragmented. Mycelial threads or acidic forest soils. The woody tissues of the forest fragments have an average diameter of about 5 floor provide an abundance of food for certain microns, which is about 5 to 10 times that of fungi that are effective decomposers of lignin. In bacteria. A gram of soil commonly contains 10 to the Sierra Nevada Mountains, there is a large 100 meters of mycelial fragments per gram. On mushroom fungus, called the train wrecker fun-this basis, the live weight of fungal tissue in soils gus, because it grows abundantly on railroad ties is about equal to that of bacteria (see Table 8.1). unless the ties are treated with creosote. The most common fungi are molds and mush- rooms. Mold mycelia are commonly seen growing on bread, clothing, or leather goods. Rhizopus is Actinomycetes a common mold that grows on bread and in soil. It Actinomycetes refers to a group of bacteria with a has rootlike structures that penetrate the bread superficial resemblance to fungi. The actino- and absorb nutrients, much like the roots of mycetes resemble bacteria in that they have a very plants. The structure on the surface of the bread simple cell structure and are about the same size forms the working structure, or mycelium, from i n cross section. They resemble filamentous fungiwhich fruiting bodies that contain spores develop. i n that they produce a branched filamentous net- work. The network compared to fungi, however, isFIGURE 8.2 A soil fungus showing mycelium and usually less extensive. Many of these organismsreproductive structures that contain spores. reproduce by spores, which appear to be very(Photograph courtesy of Michigan State University much like bacterial cells.Pesticide Research Electron Microscope Laboratory.) Actinomycetes are in great abundance in soils, as shown in Table 8.1. They make up as much as 50 percent of the colonies that develop on plates containing artificial media and inoculated with a soil extract. The numbers of actinomycetes may vary from 1 to 36 million per gram of soil. Al- though there is evidence that actinomycetes are abundant in soils, it is generally concluded they that are not as important as bacteria and fungi as decomposers. It appears that actinomycetes are much less competitive than the bacteria and fungi when fresh additions of organic matter are added to soils. Only when very resistant materials re- main do actinomycetes have good competitive ability.
12 0 SOIL ECOLOGY i nto United States from Europe. This worm makes a shallow burrow and forages on plant material at night. Some of the plant material is dragged into the burrow. In the soil, the plant materials are moistened and more readily eaten. Other kinds of earthworms exist by ingesting organic matter found in the soil. Earthworms eat their way through the soil by ingesting the soil en masse. Excreted materials (earthworm castings) are de- posited both on and within the soil. Although castings left on the soil surface of lawns are objec- tionable, earthworms feed on thatch, thus helping to prevent thatch buildup in turf grass. The inti- mate mixing of soil materials in the digestive track of earthworms, the creation of channels, and pro-FIGURE 8.3 duction of castings, alter soil structure and leave Mushroom fungal caps that containspores-an edible type. the soil more porous. Channels left open at the soil surface greatly increase water infiltration. Earthworms prefer a moist environment with an abundance of organic matter and a plentiful sup-Worms ply of available calcium. As a result, they tend toThere are two important kinds of worms in soils. be least abundant in dry and acidic sandy soils.Microscopic roundworms, nematodes, are very They normally avoid water-saturated soil. If theyabundant soil animals. They are of economic im- emerge during the day, when it is raining, they areportance because they are parasites that invade killed by ultraviolet radiation unless they quicklyliving roots. The other important worm is the ordi- find protection. Obviously, the number and activ-nary earthworm. ity of earthworms vary greatly from one location to another. Suggestive numbers of earthworms in an Ap horizon vary from a few hundred to more thanEarthworms Earthworms are perhaps the best a million per acre. Estimated weights of earth-known of the larger soil animals. The common worms are 100 to 1,000 pounds per acre (kilo-earthworm, Lumbricus terrestris, was imported grams per hectare). Attempts to increase earth- FIGURE 8.4 Distribution of microorganisms in the A, B, and C horizons of a cultivated grassland soil. All values refer to the number of organisms per gram of air-dry soil. (Data from Iowa Res. Bul. 132.)
SOIL ANIMALS 121worm activities in soils have been disappointing, the case of a root vegetable such as carrots, thebecause the numbers present probably represent market quality is seriously affected.an equilibrium population for the existing condi- I nvestigations indicate that nematode damagetions. is much more extensive than previously thought. Nematodes are a very serious problem for pineap- ple production in Hawaii, where the soil for a newNematodes Nematodes (roundworms) are mi- planting is routinely fumigated to reduce nema-croscopic worms and are the most abundant tode populations. Parasititic nematodes attack aanimals in soils. They are round shaped with a wide variety of plants throughout the world, re-pointed posterior. Under a 10-power hand lens ducing the efficiency of plant growth, analogousthey appear as tiny transparent threadlike worms, to worms living in the digestive track of animals.as illustrated in Figure 8.5. Most nematodes are Agriculturally, the most important role of nema-free-living and inhabit the water films that sur- todes in soils appears to be economic, as para-round soil particles. They lose water readily sitic consumers.through their skin, and when soils dry or otherunfavorable conditions occur, they encyst, orform a resting stage. Later, they reactivate when Arthropodsconditions become favorable. Based on food pref- A high proportion of soil animals is arthropods;erences, nematodes can be grouped into those they have an exoskeleton and jointed legs. Mostthat feed on: (1) dead and decaying organic mat- have a kind of heart and blood system, and usu-ter, (2) living roots, and (3) other living organisms ally a developed nervous system. The most abun-as predators. dant arthropods are mites and springtails. Other Parasitic nematodes deserve attention owing to i mportant soil arthropods include spiders, insectstheir economic importance in agriculture. Many (including larvae), millipedes, centipedes, woodplants are attacked, including tomatoes, peas, lice, snails, and slugs.carrots, alfalfa, turf grass, ornamentals, corn,soybeans, and fruit trees. Some parasitic nema-todes have a needlelike anterior end (stylet) used Springtails Springtails are primitive insectsto pierce plant cells and suck out the contents. less than 1 millimeter (2 i n.) long. They are dis- sHost plants respond in numerous ways: plants, tributed worldwide and are very abundant (seefor example, develop galls or deformed roots. In Table 8.1). Springtails have a springlike ap- FIGURE 8.5 A nematode and a piece of ordinary cotton thread photographed at the same magnification (270). The nematode is ,55 as thick as the thread and is not visible to the naked eye. (Courtesy H. H. Lyon, Plant Pathology Department, Cornell University.)
12 2 SOIL ECOLOGYpendage under their posterior end that permitsthem to flit or spring in every direction. If the soili n which they are abundant is disturbed and bro-ken apart, they appear as small white dots dartingto and fro. They live in the macropores of the litterlayers and feed largely on dead plant and animaltissue, feces (dung), humus, and fungal mycelia.Water is lost rapidly through the skin, so they arerestricted to moist layers. Springtails in the lowerlitter layers are the most primitive and are withouteyes and pigment. Springtails are in the order Collembola, and FIGURE 8.7 Electron microscope photograph of a they are commonly called Collembola. Their ene- mite on the top of a pinhead; magnified about 50 to mies include mites, small beetles, centipedes, 100 times. (Photograph courtesy Michigan State and small spiders. Collembola that were grown in University Pesticide Research Laboratory.) the laboratory, and are in various stages of devel- opment, are shown in Figure 8.6. todes, insect eggs, and other small animals, in- cluding springtails. Activities of mites include theMites Mites are the most abundant air- breakup and decomposition of organic material,breathing soil animals. They commonly have a movement of organic matter to deeper soil layers,saclike body with protruding appendages and are and maintenance of pore spaces (runways).related to spiders. They are small, like Collem-bola; a mite photographed on the top of a pinheadis shown in Figure 8.7. Millipedes and Centipedes Millipedes and Some mites are vegetarians and some are carni- centipedes are elongate, fairly large soil animals,vores. Most feed on dead organic debris of all with many pairs of legs. They are common inkinds. Some are predaceous and feed on nema- forests, and overturning almost any log or stone will send them running for cover. Millipedes have many pairs of legs and are mainly vegetarians.FIGURE 8.6 Springtails and eggs photographed They feed mostly on dead organic matter, butthrough a light microscope. some browse on fungal mycelia. Centipedes typi- cally have fewer pairs of legs than millipedes, and are mainly carnivorous consumers. Centipedes will attack and consume almost any-sized animal that they can master (see Figure 8.8). Worms are a favorite food of some centipedes. The data in Table 8.1 show that the numbers of millipedes and centipedes are small compared with springtails and mites, but that their biomass may be larger. White Grubs Some kinds of insect larvae playa more important role in soils than do the adults as
NUTRIENT CYCLING 12 3 animals. The excrement of animals is attacked by microbes and ingested by animals. Thus, material is subjected to decomposition in several stages and in widely different environments. Organic matter, and the soil en masse, may be ingested by earthworms, thereby producing an intimate mix- i ng of organic and mineral matter. Again, diges- tion results in some decomposition and, ulti- mately, the material is excreted as castings. The net result of these activities is the mineralization of elements in organic matter, conversion of car- bon to C0 2 , and release of energy as heat. TheFIGURE 8.8 A centipede, a common carnivorous major credit for nutrient mineralization and recyl-soil animal. i ng of nutrients and energy, however, goes to the microbes (bacteria and fungi).does, for example, the white grub. White grubsare larvae of the familar May beetle or june bug. NUTRIENT CYCLINGThe grubs are round, white, about 2 to 3 cen-timeters long, and curl into a C shape when dis-turbed. The head is black and there are three pairs Nutrient cycling is the exchange of nutrient ele-of legs just behind the head. White grubs feed ments between the living and nonliving parts ofmainly on grass roots and cause dead spots in the ecosystem. Plants and microbes absorb nutri-l awns. A wide variety of other plants are also ents and incorporate them into organic matter,attacked, making white grubs an important agri- and the microbes (with aid of animals) digest thecultural pest. Moles feed on insect larvae and organic matter and release the nutrients in min-earthworms, resulting in a greater likelihood of eral form. Nutrient cycling conserves the nutrientmole damage in lawns when white grubs are supply and results in repeated use of the nutrientspresent. in an ecosystem. Ants and termites are important soil insects and Nutrient Cycling Processeswill be considered later in relation to the earth-moving activities of soil animals. Two simultaneous processes, mineralization andI nterdependence of Microbes and immobilization, are involved in nutrient cycling.Animals in Decomposition Immobilization is the uptake of inorganic ele- ments (nutrients) from the soil by organisms andMicroorganisms and the animals work together as conversion of the elements into microbial anda decomposing team. When a leaf falls onto the plant tissues. These nutrients are used for growthforest floor, both micoorganisms and animals at- and are incorporated into organic matter. Mineral-tack the leaf. Holes made in the leaf by mites and ization is the conversion of the elements in or-springtails facilitate the entrance of microor- ganic matter into mineral or ionic forms such as Cat-,ganisms inside the leaf. Soil animals also break NH 3 , H2P0 4 - , S0 42- , and K+. These ionsup organic debris and increase the surface area then exist in the soil solution and are available forfor subsequent attack by bacteria and fungi. Soil another cycle of immobilization and mineral-animals ingest bacteria and fungi, which continue ization.to function within the digestive tracks of the Mineralization is a relatively inefficient process
12 4 SOIL ECOLOGYi n that much of the carbon is lost as CO 2 andmuch of the energy escapes as heat. This typicallyproduces a supply of nutrients that exceeds theneeds of the decomposers; the excess of nutrientsreleased can be absorbed by plant roots. Only inunusual circumstances is there serious compe-tition between the plants and microbes for nu-trients.A Case Study of Nutrient CyclingOne approach to the study of nutrient cycling is toselect a small watershed and to measure changesi n the amounts and locations of the nutrients overti me. An example is a 38-acre (15-hectare) water-shed in the White Mountains of New Hampshire.In this mature forest, the amount of nutrientstaken up from the soil approximated the amountof nutrients returned to the soil. For calcium, thiswas 49 kilograms per hectare (44 lb per acre)taken up from the soil and returned to the soil, as illustrated in Figure 8.9. About 9 kilograms of cal-cium per hectare were added to the system by mineral weathering and 3 kilograms per hectarewere added in the precipitation. These additions FIGURE 8.9 Calcium cycling in a hardwood forestwere balanced by losses due to leaching and run- in New Hampshire in kilograms per hectare per year. off. This means, in effect, that 80 percent of the (Data from Bormann and Likens, 1970.) calcium in the cycle was recycled or reused by the Effect of Crop Harvesting on forest each year. Plants were able to take up 49 Nutrient Cycling kilograms of calcium per hectare, whereas only 12 kilograms were added to the system annually. Researchers concluded that northern hardwood The grain and stalks (stover) of a corn (Zea forests have a remarkable ability to hold and to maize) crop may contain nitrogen equal to more recirculate nutrients. than 200 kilograms per hectare (200 lb per acre). It The data in Table 8.2 show that 70 to 86 percent is not uncommon for farmers in the Corn Belt of four major nutrients absorbed from the soil region of the United States to add this amount of were recycled by return of the nutrients in leaves nitrogen to the soil annually to maintain high and wood to the litter layer each year. Without this yields. According to the data in Table 8.2, by con- extensive recycling, the productivity of the forests trast, only 10 kilograms of nitrogen per hectare would be much lower. Nutrient cycling accounts were stored annually in wood or removed from for the existence of enormous forests on some the system by tree growth in a beech forest. Natu- very infertile soils. In some very infertile soils, rally occurring nitrogen fixation and the addition most of the nutrients exist in the organic matter via precipitation can readily supply this amount of and are efficiently recycled. nitrogen. Basically, food production represents
SOIL MICROBE AND ORGANISM INTERACTIONS 12 5 TABLE 8.2 Annual Uptake, Retention, and Return of Nutrients to the Soil in a Beech Forest Stoeckler, J. H., and H. F. Arneman, "Fertilizers in Forestry," Adv. in Agron. 12:127-195, 1960. By permission of author and Academic Press.nutrient harvesting. The natural nutrient cycling tion of manure to the land. Waste disposal is anprocesses cannot provide enough nutrients to important problem. Conversely, large crop farmsgrow high-yielding crops. Yields of grain in many without animals have no manure to apply to theof the worlds agricultural systems stabilize at land, so they balance the nutrient cycle deficitonly 500 to 600 kilograms per hectare (8 to 10 with the application of fertilizer.bushels per acre) where there is little input ofnutrients as manure or fertilizer. SOIL MICROBE AND When the agricultural system is based on feed- ORGANISM INTERACTIONSi ng most of the crop production to animals, as indairying, most of the nutrients in the feed areexcreted by the animals as manure. On the aver- The role of soil organisms in nutrient cycling hasage, 75 to 80 percent of the nitrogen, 80 percent of been stressed, and it has been shown that somethe phosphorus, and 85 to 90 percent of the po- organisms are parasites that feed on plant roots.tassium in the feed that is eaten by farm animals is In this section the specialized roles of microor-excreted as manure. Crops can be harvested with ganisms living near and adjacent to plant rootsonly a modest drain on the nutrients in the cycle if will be considered.the nutrients in the manure are returned to thel and. Careful management of manure and the useof legumes to fix nitrogen, together with lime to The Rhizospherereduce soil acidity, were the most important prac- Plant roots leak or exude a large number of or-tices used to develop a prosperous agriculture in ganic substances into the soil. Sloughing of rootdairy states like New York and Wisconsin. caps, and other root cells, provide much new Specialization in agriculture has resulted in organic matter. More than 25 percent of the photo-very large crop and animal farms. Today, many synthate produced by plants may be lost from thelarge livestock enterprises have more manure, roots to the soil. These substances are food forand nutrients, than they can dispose of by applica- microorganisms, and they create a zone of in-
12 6 SOIL ECOLOGYtense biological activity near the roots in an area ganisms quickly die when added to soils. Somecalled the rhizosphere. The rhizosphere is the human disease organisms, however, are very per-zone of soil immediately adjacent to plant roots in sistent. The disposal of human feces by back-which the kinds, numbers, or activities of micro- packers in wilderness areas poses a health threat,organisms differ greatly from that of the bulk soil. because some organisms are not readily killed byAlthough many kinds of organisms inhabit the shallow burial. There is also danger of causingrhizosphere, it appears that bacteria are benefited disease in humans when human feces are usedthe most. The bacteria appear mainly in colonies for fertilizing gardens or fields, because of resis-and may cover 4 to 10 percent of the root surface. tant spore forms of some disease organisms. InThe intimate association of bacteria and roots China, much of the human excrement is used formay result in mutually beneficial or detrimental fertilizer, but this material is composted for a suf-effects. Nutrient availability may be affected and ficiently long period of time to kill the diseaseplant-growth-stimulating or toxic substances may organisms before the compost is applied to thebe produced. Since roots tend to grow better in fields.sterile soil inoculated with organisms than in ster- Historically, human excrement (night soil) hasile soil that is not inoculated, it appears that the played an important role in the maintenancenet effect of bacteria growing in the rhizosphere is of soil fertility. In 1974 the value of the nitro-beneficial. gen, phosphorus, and potassium in the human excreta in India was estimated to be worth moreDisease than $700 million as compared with the cost of the nutrients as fertilizer. In countries where most ofSoils may contain organisms that cause both the food is consumed by humans, careful use ofplant and animal disease. Bacteria are responsi- human excrement can be very helpful in main-ble for wilt of tomatoes and potatoes, soft rots of a taining soil fertility. In highly industrialized soci-number of vegetables, leaf spots, and galls. Some eties, the nutrients are disposed of through sew-fungi cause damping-off of seedlings, cabbage age systems and lost from the food productionyellows, mildews, blight, and certain rusts. The enterprise. The low cost of energy means that thecatastrophic potato famine in Ireland from 1845 to cost of nutrients in the form of fertilizer is less 1846 was caused by a fungus that produced po- expensive than processing waste to recover andtato blight. Certain species of actinomycetes reuse the nutrients.cause scab in potatoes and sugar beets. The soil Mycorrhizaused for bedding and greenhouse plants is rou-tinely treated with heat or chemicals to kill plantpathogens. Some fungi infect the roots of most plants. Fortu- The organisms that produce disease in humans nately, most of the fungi form a symbiotic rela-must be able to tolerate the soil environment, at tionship-one that benefits both fungi and higherleast for a period long enough to cause infection. plants. After the appropriate fungal spores germi-Many disease organisms occur in fecal matter and nate, hyphae, or small segments of the mycelium,human sewage, such as the virus that produces i nvade young roots and grow a mycelium both oni nfectious hepatitis and the protozoan (amoeba) the exterior and interior of roots. Fungal hyphaethat produces amebic dysentery. When materials on the exterior of roots serve as an extension ofcontaining animal disease organisms are added roots for water and nutrient absorption. Theseto soil, the organisms encounter a very different external hyphae function as root hairs and areenvironment than the one they occupied in an called mycorrhiza. Mycorrhiza literally meansanimal. Consequently, many animal disease or- "fungus roots."
SOIL MICROBE AND ORGANISM INTERACTIONS 127 There are two types of hypha or mycorrhiza: tive surface area of roots for the absorptionectotrophic and endotrophic. The ectotrophic hy- of water and nutrients, particularly phosphorus;phae exist between the epidermal cells, using (2) increased drought and heat resistance; andpectin and other carbohydrates for food. Contin- (3) reduced infection by disease organisms.ued growth of the hyphae outside the root results Nitrogen Fixationi n the formation of a sheath (mantle) that com-pletely surrounds the root, as shown in Figure8.10. Literally thousands of species belong in the We live in a "sea" of nitrogen, because the atmo-group of fungi that produce ectotrophic mycor- sphere is 79 percent nitrogen. In spite of this,rhiza and produce mushroom or puffball fruiting nitrogen is generally considered the most limitingbodies. They are associated typically with trees nutrient for plant growth. Nitrogen exists in the airand, thus are beneficial to trees, as illustrated in as N 2 and, as such, is unavailable to higher plants Figure 8.11. and most soil microbes. There are some species Endomycorrhiza are more abundant than ecto- of bacteria (nitrogen fixers) that absorb N 2 gas mycorrhiza. They benefit most of the field and from the air and convert the nitrogen into ammo-vegetable crop plants. Hyphae invade roots and nia that they and the host plant can use. This ramify both between and within cells, usually process of nitrogen fixation is symbiotic. The bac- avoiding the very central core of the roots. The teria obtain food from the host plant and the host host plants provide the fungi with food. The bene- plant benefits from the nitrogen fixed. The bac- fits to the host plant include: (1) increased effec- teria responds to and invades the roots of the hostFIGURE 8.10 Ectomycorrhiza. (a) Uninfected pine root. (b) I nfected rootwith fungal mantle. (c) Diagram of the anatomy. The Hartig net is the hyphaebetween root cells. (Courtesy D. H. Marx, University of Georgia.)
12 8 SOIL ECOLOGY Pesticide Degradation Pesticides include those substances used to con- trol or eradicate insects, disease organisms, and weeds. One of the first successful synthetic pesti- cides, DDT, was used to kill mosquitoes for ma- l aria control. After decades of use it was found in the cells of many animals throughout the world. This dramatized the resistance of DDT to biode- gradation and its persistence in the environment. The structure of DDT appears to be different from naturally occurring compounds and, therefore, few if any organisms have enzymes that can degrade DDT. The difference in structure and de-FIGURE 8.11 Effect of mycorrhizal fungi on the gradability of 2,4-D and 2,4,5-T is illustrated ingrowth of 6-month-old Monterey pine seedlings. Figure 8.13.(a) Fertile Prairie soil. (b) Fertile Prairie soil plus 0.2 Generally, the pesticides used have almost nopercent by weight of Plainfield sand from a forest. significant effect on soil microbes. The soil mi-(c) Plainfield sand alone. Only plants of b and c were crobes attack the organic pesticides similar toi nfected with mycorrhiza. (Photograph courtesy of thelate Dr. S. A. Wilde, University of Wisconsin.) their attack of other organic substrates. In some i nstances, the microbes detoxify the pesticide, for example, by the shifting of chlorine atoms. Inplant, and the host responds by forming a nodule other cases, the pesticide is metabolized or de-that surrounds the bacteria, and in which nitrogen graded. In general, those pesticides that resistis fixed, as shown in Figure 8.12. detoxification and/or degradation and persist in I n aquatic ecosystems, blue-green algae fix ni- the environment long after they are applied, aretrogen. They are the land counterpart by being the called hard pesticides. The others that degrademajor means by which nitrogen from the atmo- easily are soft pesticides. The challenge for scien-sphere is added to aquatic ecosystems. A detailed tists is to develop pesticides that can perform theirconsideration of nitrogen fixation occurs in Chap- useful function and disappear from the ecoystemter 12, where the nitrogen cycle is discussed. with minimal undesirable side effects. Oil and Natural Gas DecontaminationSOIL ORGANISMS ANDENVIRONMENTAL QUALITY Many species of microbes can decompose petro- l eum compounds. Crude oil and natural gas are aDuring the billions of years of organic evolution, good energy source, or substrate, for many mi-organisms evolved that could decompose the crobes, and their growth is greatly stimulated.compounds naturally synthesized in the environ- The first result of an oil spill or natural gas leakment. Today, humans have become important is the displacement of soil air and creation of ancontributors of both natural and synthetic com- anaerobic soil. Any vegetation is likely to be killedpounds to the environment. The problems of pes- by a lack of soil oxygen. The reducing conditionsticide degradation and contamination of soils result in large increases in available iron andfrom oil spills will be considered as they relate to manganese. It is suspected that toxic levels ofenvironmental quality. The application of sewage manganese for higher plants are produced insludge to the land is covered in Chapter 15. some cases. In time, the oil or natural gas is
SOIL ORGANISMS AND ENVIRONMENTAL QUALITY 129FIGURE 8.12 Symbiotic nitrogen-fixing nodules on the roots of soybeanroots (left), and on sweet clover roots (right). (Photographs courtesy NitraginCompany.)FIGURE 8.13 Structure representing 2,4-D on the decomposed and the soil returns to near normall eft and 2,4,5-T on the right. The structures are very conditions. A study of 12 soils exposed to con-similar except for the additional C1 at the meta tamination near Oklahoma City showed that or-position of 2,4,5-T, which is metabolized with greatdifficulty (if at all) by soil microorganisms. ganic matter and nitrogen contents had been in- creased about 2.5 times. The increased nitrogen supply in soils that had been contaminated proba- bly explains the increased crop yields obtained on old oil spills. Natural gas leaks are common in cities. The destruction of grass and trees is due to a lack of normal soil air, which is displaced by the natural gas (methane). Sometimes the problem can be diagnosed by detecting the odor of a pungent compound in the natural gas that escapes from a crack in a nearby sidewalk or driveway. Because
13 0 SOIL ECOLOGYmethane (CH 4) is odorless, an odor has been within the soil, depositing it on the surface. Someadded to public gas supplies. of the largest ant mounds are about 1 meter tall and more than 1 meter in diameter. The effect of this transport is comparable to that of earthwormsEARTH MOVING BY SOIL ANIMALS i n creating thick A horizons and in burying objects lying on the soil surface.All soil animals participate as consumers and A study of ant activity on a prairie in southwest-play a minor role in the cycling of nutrients and ern Wisconsin showed that ants brought materialenergy. Many of the larger animals move soil to to the surface from depths of about 2 meters andsuch an extent that they affect soil formation. built mounds about 15 centimeters high and more than 30 centimeters in diameter (see Fig. 8.14).Earthworm Activity Furthermore, it was estimated that 1.7 percent of the land was covered with ant mounds. AssumingEarthworms are probably the best known earth that the average life of a mound is 12 years, themovers. Darwin made extensive studies and entire land surface would be reoccupied everyfound that earthworms may deposit 4 to 6 metric 600 years. The researchers believe that the earthtons of castings per hectare (10 to 15 tons per movement activity of ants resulted in a greateracre) annually on the soil surface. This could than normal content of clay in the A horizon,result in the buildup of a 2.5-centimeter-thick because of the movement to the surface of mate-layer of soil every 12 years. This activity produces rial from a Bt horizon, a clay-enriched B horizon.thicker than normal, dark-colored surface layers Ants also harvest large amounts of plant mate-i n soils. Stones and artifacts are buried over time, rial and in some ecosystems are important con-which is a subject of great interest to archae- sumers. Their gathering of seeds retards naturalologists. reseeding on range lands. Leaf harvester ants As a result of their earth-moving activities, march long distances to cut fragments of leavesearthworms leave channels. Where these chan- and stems and bring them to their nests to feednels are open at the soil surface, they can tranport fungi. The fungi are used as food. Organic matterwater very rapidly into and through the soil. Ear- is incorporated into the soil depths and nutrients lier it was reported that infiltration of water from concentrate in the nest sites. irrigation furrows increased after earthworms had Termites inhabit tropical and subtropical soils.burrowed to and entered the irrigation furrows. They exhibit great diversity in food and nesting Renewed interest in the role of earthworms has habits. Some feed on wood, some feed on organicbeen created in regard to no-till systems, which refuse, and others actively cultivate fungi. Pro- leave plant residues on the soil surface. The resi- tozoa (single-celled microscopic animals) in thedues provide a source of food for the earthworms, digestive tract of many termites aid in the diges-which forage on the soil surface, and the residues tion of woody materials. Some species build hugeand growing plants protect the soil surface from nests up to 3 meters high and several meters inraindrop impact so that the channels remain open diameter. Most mounds are on a smaller scale.and functional for water infiltration. Some termites have nests in the soil and tunnnels on the surface to permit foraging for food. SoilAnts and Termites material is brought to the surface from depths as great as 3 meters. Termites have had an enormousThe activities of ants and termites are, perhaps, effect on many tropical soils, where their earth-more important than the activities of earthworms. moving activity has been occurring for hundredsAnts transport large quantities of material from of thousands of years.
SUMMARY 131 ents occurs at the nest sites, which is why some farmers in Southeast Asia make use of the higher soil fertility in areas occupied by mounds. Rodents Many rodents, including mice, ground squirrels, marmots, gophers, and prairie dogs inhabit the soil. A characteristic microrelief called mima mounds, which consists of small mounds of earth, is the work of gophers in Washington and California. Mima mounds occur on shallow soils. They appear to be the result of gophers building nests in the dry soil near the tops of the mounds. Successive generations of gophers at the same site create mounds that range from 0.5 to 1 meter high and more than 5 to 30 meters in diameter. Extensive earth moving by paririe dogs has been documented. An average of 42 mounds per hectare (17 per acre) were observed near Akron, Colorado. Each mound consisted of an average of 39 tons of soil material. The upper 2 to 3 meters of the soil formed from loess, a silty wind-deposited material, and was underlain by sand and gravel. All of the mounds observed contained sand and gravel that had been brought up from depths of more than 2 meters. Prairie-dog activity had changed the surface soil texture from silt loam to loam on one third of the area. Abandoned burrows filled with dark-colored surface soil, called crotovinas, are common in grasslands.FIGURE 8.14 Ant (Formica cinera) in a Prairie soili n southwestern Wisconsin. Upper photo shows ant SUMMARYmounds more than 15 centimeters high and morethan 30 centimeters in diameter. The lower sketchshows soil horizons and location of ant channels;numbers refer to the number of channels observed at Higher plants are the major producers contribut-the depth indicated. (Photographs courtesy Dr. F. D. i ng to the supply of soil organic matter. The mi-Hole, Wisconsin Geological and Natural History croorganisms (bacteria and fungi) are the majorSurvey, University of Wisconsin.) decomposers and are mainly responsible for the cycling of nutrients and energy in soil eco- In summary, ants and termites create channels systems.i n soils and transport soil materials to such an Soil animals play a minor role in the cycling ofextent that soil horizons are greatly affected and nutrients and energy, but play an important role insometimes obliterated. A concentration of nutri- earth-moving activities.
13 2 SOIL ECOLOGY Nutrient cyling results in reuse of the nutrients Agriculture." Adv. in Agronomy. 13:249-268. Aca-i n an ecosystem. Nutrients are efficiently recycled demic Press, New York.i n natural ecosystems. Interference of the cycle, Baxter, F. P. and F. D. Hole. 1967. "Ant Pedoturbation insuch as cropping and removal of nutrients in a Prairie Soil." Soil Sci. Soc. Am. Proc. 31:425-428.food, results in reduced soil fertility. Manures and Bormann, F. F. and G. E. Likens. 1970. "The Nutrient Cycles of an Ecosystem." Sci. Am. 223:92-101.fertilizers are used to maintain soil fertility in agri- Bowen, E. 1972. "The High Sierra." Time, Inc., Newculture. York Soil organisms and higher plants engage in Buol, S. W., F. D. Hole, and R. J. McCracken. 1973. Soilmany interactions related to disease, mycorrhiza, Genesis and Morphology. I owa State Universityand nitrogen fixation, and soil organisms and Press, Ames.higher plants compete for the same growth Ellis, R., Jr. and R. S. Adams, Jr. 1961. "Contaminationfactors. of Soils by Petroleum Hydrocarbons." Adv. in A zone adjacent to plant roots with a high popu- Agronomy. 13:197-216. Academic Press, New York.l ation of microorganisms is the rhizosphere. Flaig, W., B. Nagar, H. Sochtig, and C. Tietjen. 1977. Microorganisms play important environmental Organic Materials and Soil Productivity. FAO Soils Bull. 35. United Nations, Rome.quality roles, such as detoxification of chemicals Richards, B. N. 1974. Introduction to the Soil Ecosys-and decomposition of oil from spills. tem. Longmans, New York. Earthworms, ants, termites, and rodents move Thorp, J. 1949. "Effect of Certain Animals that Live inl arge quantities of soil and may greatly alter the Soils." Sci. Monthly. 42:180-191.nature of soil horizons. Schaller, F. 1968. Soil Animals. University of Michigan Press, Ann Arbor. Smucker, A. J. M. 1984. "Carbon Utilization and Losses by Plant Root Systems." Roots, Nutrient and WaterREFERENCES Flux and Plant Root Systems. ASA Spec. Pub. 49, Soil Sci. Soc. Am., Madison.Alexander, M. 1977. Soil Microbiology, 2nd ed. John Stoeckeler, J. H. and H. F. Arneman. 1960. "Fertilizers in Wiley, New York. Forestry." Adv. in Agronomy. 12:127-195.Arkley, R. J. and H. C. Brown. 1954. "The Origin of Mima Temple. K. L. A. K. Camper, and R. C. Lucas. 1982. Mound Microrelief in the Far Western States." Soil "Potential Health Hazard from Human Wastes in Wil- Sci. Soc. Am. Proc. 18:195-199. derness." J. Soil and Water Con. 37:357-359.Barley, K. P. 1961. "The Abundance of Earthworms in Walters, E. M. P. 1960. Animal Life in the Tropics. Agricultural Land and Their Possible Significance in George Allen and Unwin, London.
CHAPTER 9 SOIL ORGANIC MATTERAlmost all life in the soil is dependent on organic there is a continual growth of plants and additionsmatter for nutrients and energy. People have long to these three pools of organic matter: standingrecognized the importance of organic matter for crop, forest floor, and soil. These three pools ofplant growth. Although organic matter in soils is organic matter represent a supply of nutrients forvery beneficial, Justus von Liebig, a famous Ger- future growth.man chemist, pointed out about 150 years ago In grassland ecosystems, much more of thethat soils composed entirely of organic matter are organic matter is in the soil and much less occursnaturally infertile. This chapter considers the ori- i n the standing plants and grassland floor. Al-gin, nature, and importance of the organic matter though approximately 50 percent of the total or-i n soils. ganic matter in the forest ecosystem may be in the soil, over 95 percent may be in the soil, where grasses are the dominant vegetation.THE ORGANIC MATTER IN ECOSYSTEMS DECOMPOSITION AND ACCUMULATIONThe organic matter in an ecosystem consists ofthe organic matter above and below the soil sur-face. The distribution of organic matter in a pon- The primary or original source of soil organicderosa pine forest ecosystem is shown in Table matter (SOM) is the production of the primary9.1. The distribution of organic matter, expressed producers-the higher plants. This organic mate-as organic carbon, was 38 percent in the trees and rial is subsequently consumed and decomposedground cover and 9 percent in the forest floor. The by soil organisms. The result is the decom-remaining 53 percent of the organic matter was in position and the accumulation of organic matterthe soil and included the roots plus the organic i n soils that has great diversity and a highly vari-matter associated with soil particles. In the forest able composition. 133
134 SOIL ORGANIC MATTER TABLE 9.1 Quantity and Distribution of Organic Carbon (organic matter) in a Ponderosa Ecosystem in Arizona Organic Carbon Adapted from Klemmedson, 1975.Decomposition of Plant Residues ble fraction, cellulose and hemicellulose, and other readily degradable compounds, there was a corresponding and rapid increase in microbialThe plant residues, including tops and roots, con- products (see Figure 9.1b).tain a wide variety of compounds: sugar, cellu-l ose, hemicellulose, protein, lignin, fat, wax, andothers. Observe from Table 9.2 that the composi- Microbial products include living and dead mi-tion of SOM is distinctly different than that of the crobial cells and their waste or excretion prod-original plant material added to the soil. ucts. Some of the organic compounds that are In an experiment, wheat straw was added to synthesized in the soil during decomposition re-soil and the changes in the major components in act with each other and with mineral soil com-the straw were followed over time. It was found ponents. Consequently, the decomposition ofthat proteins, the soluble fraction, and the cellu- plant residues results in: (1) the production ofl ose and hemicellulose disappeared or decom- a considerable mass of microbial products, andposed very rapidly, whereas lignin decomposed (2) the production of a wide variety of materials ofvery slowly, as shown in Figure 9.1a. As a result of varying resistance to decomposition. As a resultthe rapid decomposition of proteins and the solu- of the addition and decomposition of plant and animal residues in soils, labile and stable frac- tions of organic matter are produced. Labile or-TABLE 9.2 Partial Composition of Mature Plant ganic matter is organic matter that is liable toTissue and Soil Organic Matter change or is unstable. The liable and stable or- ganic matter fractions correspond, in general, to the organic residues and humus fractions, respec- tively. Labile Soil Organic Matter The labile fraction of SOM consists of any readily degradable materials from the plant and animal residues, and readily degradable microbial prod-
DECOMPOSITION AND ACCUMULATION 13 5 FIGURE 9.1 Decomposition of wheat straw in soil in the laboratory. (a) Decreases over ti me: proteins and solubles (C1), cellulose and hemicellulose (C2), and lignin (C3). (b) Changes over time in amount of carbon in original plant material and in microbial products. (Data from Van Veen and Paul, 1981.)ucts. The labile organic matter composes about studies have shown that certain stable SOM frac-10 to 20 percent of the total SOM. Labile organic tions are commonly more than a few hundred ormatter is an important reservoir of nutrients be- thousands of years old. The stable organic mattercause the nutrients, especially nitrogen, are rap- has a long residence time in the soil and playsi dly recycled in the soil ecosystem. The addition i mportant roles in structure formation and sta-to the soil of a relatively small amount of easily bility, water adsorption, and adsorption of nutri-degraded organic matter, such as manure, or- ent cations.ganic waste materials, and plant residues, may Stable soil organic matter, or humus, originatescontribute more to the available nutrient supply from several processes. Lignin, a 6-carbon ringthan the slow release of nutrients from the much structure, is a plant compound and is very resis-larger amount of stable organic matter. The labile tant per se. Thus, lignin accumulates in soils be-organic matter is rapidly degraded when condi- cause of its resistant or stable nature. Organictions are favorable. As a result, any labile organic compounds of varying resistance interact with lig-matter incorporated within soils tends to disap- nin and form new compounds that are resistant.pear quickly if conditions are favorable for de- This accounts for the significant quantity ofcomposition. Therefore, the decomposers are fre- protein or proteinlike material in humus (see Ta-quently quite inactive, owing to the rapid ble 9.2).disappearance of readily decomposable sub- Much of the stable organic matter is intimatelystrates or labile organic matter. associated with the mineral particles. As such, stable organic matter plays an important role in the formation of soil structure and the ability ofStable Soil Organic Matter soil peds to resist crushing and breakdown fromThe stable SOM fraction consists of resistant com- tillage and raindrop impact. Studies have shownpounds: (1) in the decomposing residues, (2) in that some readily decomposable materialsmicrobial products, and (3) that formed as a re- (sugars and proteins) are decomposed by mi-sult of interaction of organic compounds with crobes in less than a day under laboratory condi-each other and with the mineral components of tions. When the same materials are added tothe soil, especially the clay. The stable organic soils, the decomposition time is greatly in-matter is equivalent to humus. The nutrients creased. The adsorption of organic compoundswithin it are recycled very slowly. Radioactive onto soil, particles makes these compounds less
13 6 SOIL ORGANIC MATTERdecomposable. Scientists believe that the organic association of mineral and organic matter thatmatter is protected from decomposition; this protects the organic matter from decompositionprotection helps to increase the stability of both and forms stable soil peds. One half or more ofreadily decomposable and resistant organic mat- the SOM is likely to be protected.ter. Much of the humus is associated intimately Decomposition Rateswith clay and silt particles, and the humus playsan important role in formation of soil structure, as Although SOM has been characterized as havingshown in Figure 9.2. two forms-labile and stable-there is great vari- Polysaccharides include microbial gum and ety in the nature and decomposition rates of vari-two other abundant compounds found in micro- ous organic fractions of the SOM. In a nativebial cell walls: hemicellulose and cellulose. grassland soil, about 9 percent of the SOM.wereThese polysaccharides are organic polymers (gi- shown to consist of roots and plant residues and 4ant organic moleclules formed by the combina- percent consists of unprotected and decomposa-tion of smaller molecules) having high molecular ble liable organic matter with a decompositionweight and are present as ropes and nets. These rate of 8 percent per day. This rate, which as-polymers are widely and intimately associated sumes optimal conditions, is shown in Table 9.3.with the soil fabric. On drying, soil particle sur- Soil organisms were 1.3 percent of the total or-faces are brought into very close contact with the ganic matter with a decomposition rate of 3 per-netlike arrangement of the organic compounds. cent per day.The hydroxyls of the polymers and the exposed The other fractions of the SOM, 84 percent, hadoxygen atoms of the mineral soil particles bind much slower decomposition rates. The bulk ofstrongly to each other. The result is an intimate the SOM (54 percent) consisted of decomposable organic matter that was protected by attachment to clays and other mineral components, and had aFIGURE 9.2 Some arrangements of organic matter decomposition rate of 0.08 percent per day. Twoand mineral soil particles. Organic matter intimately fractions, composed mainly of ligneous material,associated with mineral particles is protected from decomposed at 0.0008 and 0.000,008 percent perdecomposition and contributes to the formation and day, for unprotected and protected ligneous mate-stability of soil structure. (a) Quartz-organicmatter-quartz. (b) Quartz-organic matter-clay. rials, respectively. Thus, the decomposition rate(c) Organic matter-clay-organic matter. of the most readily decomposable material is about 1 million times that of the most stable (see Table 9.3). The most resistant organic matter frac- tions have been called recalcitrant, meaning stubborn. Assuming that plant residues, roots, decom- posable unprotected organic matter, and soil or- ganisms are liable organic matter, the data in Table 9.3 show that the soil contained about 15 percent liable organic matter and 85 percent sta- ble organic matter. Properties of Stable Soil Organic Matter Stable soil organic matter (humus) is a heteroge- neous mixture of amorphous compounds that are resistant to microbial decomposition and possess
DECOMPOSITION AND ACCUMULATION 137 TABLE 9.3 Soil Organic Matter and Decomposition Rates under Optimum Conditions for a Native Grassland Soil Organic carbon, upper 16 inches Adapted from Van Veen and Paul, 1981.a large surface area per gram. The large surface negative charge. Cations in the soil solution tendarea enables humus to absorb water equal to to be weakly adsorbed onto the negativelymany times its weight. charged phenolic and carboxyl groups. However, During decomposition of plant and animals adsorbed cations can be readily exchanged byresidues, there is a loss of carbon as carbon diox- other cations remaining in solution. The total neg-ide and the conservation and reincorporation of ative charge of the SOM, or ability to adsorb ex-nitrogen into microbial products, which eventu- changeable cations, is called the cation exchangeally are incorporated into humus. As a conse- capacity (CEC). The CEC of organic matter actsquence, humus contains about 50 to 60 percent similarly to that of clay particles, in that the chargecarbon and about 5 percent nitrogen, producing a adsorbs and holds nutrient cations in position toC : N ratio of 10 or 12 to 1. The more decomposed be available to plants, while effectively holdingthe humus is the lower or narrower will be the these cations against loss by leaching. The mostC : N ratio. Compared with plant materials, hu- abundant nutrient ions adsorbed are Ca.²1 , M g ²+ ,mus is relatively rich in nitrogen and, when miner- and K + . Adsorbed H + affects the pH of the soilalized, humus is a good source of biologically solution, depending on the amount adsorbed andavailable nitrogen. Humus is also a significant the degree of dissociation into the soil solution.source of sulfur and phosphorus. The ratio of Clay and SOM are the source of most of theC:N:P:S is about 100:10:1:1. negative charge in soils. The sum of the CEC of the Humus contains phenolic groups, 6-C rings SOM plus the CEC of the clay is generally con-with OH, that dissociate H + at pH above 7.0. sidered equivalent to the CEC of the total soil. TheDuring humus formation there is an increase in source and amount of CEC of soil clays are dis-carboxyl groups, R-COOH, and these groups tend cussed in Chapters 10 and 11.to dissociate H+ at pH less than 7: R-COOH = R-C00 - + H+ Protection of Organic Matter by Clay The amount of organic matter present in a soil atAs a result, humus has a considerable amount of any given time is the net effect of the amount that
13 8 SOIL ORGANIC MATTERhas been added minus the amount that has beendecomposed. The major factors affecting the SOMcontent are soil texture, vegetation type, watercontent or aeration of the soil, and soil temper-ature. Locally, within a given landscape, a correlationhas been found between soil texture and the or-ganic matter content of soils as a result of the roleof the soil matrix, especially clay, in protectingorganic matter from decomposition. Organic mat-ter is positively correlated with the clay contentand negatively correlated with the sand content insome grassland soils in Wyoming (see Figure9.3). These soils had a mean average soil temper-ature of 8 to 15° C (47 to 59° F). In the same study, soils with lower average FIGURE 9.4 Model of soil nitrogen variation of thetemperature showed a weaker relationship be- Great Plains from Canada to Mexico (back to fronttween organic matter content and texture. This curves) and from the desert to the humid regionssuggests that low soil temperature, by decreasing (left to right curves). The NSQ is the precipitationthe rate of decomposition, appeared to have had divided by the absolute saturation deficit of air; it is aan important effect on the organic matter content measure of the effectiveness of the precipitation. Decreases in temperature and increases in humidityof the soils. The data in Figure 9.4 are generally are associated with increases in total soil-nitrogenrepresentative of the plains of the United States content. (From Jenny and used by permission ofbetween Canada and Mexico, and show that high Springer-Verlag, New York, copyright (D 1980.)precipitation at low temperature results in muchmore organic matter in soils as compared to highprecipitation and high temperature. In most local ble clay content tend to have a higher content of(farm) situations, where temperature is a con- organic matter and, therefore tend to have a goodstant, clay content is positively associated with combination of soil fertility and physicalorganic matter content. Thus, soils with apprecia- properties. FIGURE 9.3 Relationships between the content of organic matter to a depth of 40 centimeters and the clay and sand content of some Wyoming grassland soils. (Adapted from McDaniel and Munn, 1985.)
ORGANIC SOILS 139ORGANIC SOILS ganic soil materials that are of a significant thick- ness, depending on the depth to rock and otherThe emphasis thus far has been on mineral soils, lithic materials.soils with a volume composition consisting ofabout 50 percent solids, and these solids com-posed of 45 percent mineral matter and 5 percent Formation of Organic Soilsorganic matter. In such soils the mineral fraction High organic matter contents in soils are the re-has a dominating influence on soil properties. sult of slow decomposition rates rather than highOrganic soils, by contrast, have properties that are rates of organic matter addition. Organic soils arecontrolled mainly by the organic matter. very common in tundra regions where the rate of plant growth is very slow. In the shallow water ofOrganic Soil Materials Defined lakes and ponds, plant residues accumulate in- stead of decomposing because of the anaerobicOrganic soil materials are saturated with water for conditions. Consequently, organic soils may con-long periods or are artificially drained and, ex- sist almost entirely of organic matter. In somecluding roots, (1) contain 18 percent (dry-weight cases, an infusion of mineral sediment from soilbasis) organic carbon if the mineral fraction is 60 erosion may result in the accumulation of somepercent or more clay; (2) contain 12 percent or mineral matter, together with the organic matter.more organic carbon if the mineral fraction has no Some organic soils, peat, for example, are minedclay; and (3) contain a proportional content of and used for fuel.organic carbon between 12 and 18 percent, de- Formation of many organic soils consists of thepending on clay content, as shown in Figure 9.5. If accumulation of organic matter in a body of wa-the soils are never water saturated for more than a ter. After glaciation, many depressions receivedfew days each year, the organic carbon content clay and other materials that sealed the bottomsmust be 20 percent or more. and created ponds and lakes. As the plant mate- Organic soils are those soils composed of or- rial accumulated on the bottom of the bodies of FIGURE 9.5 Organic carbon content (dry weight basis) of organic and mineral soil materials. Mineral soil materials below the heavy solid line and organic soil materials above the heavy solid line. Organic soil materials above the dashed line are water saturated for no more than a few days per year.
140 SOIL ORGANIC MATTERwater, the water depth decreased and the kinds ofplants contributing organic matter changed. An-other important organic component is the re-mains and fecal products of small aquaticanimals. Minor climatic changes have occurred sincethe last glaciation, and this has had an effect onthe water levels and types of vegetation and theanimal populations. Thus, a series of layers or soilhorizons develop from the bottom upward thatreflect the conditions at the time each layer wasformed. Pollen grains are preserved in the peat,which can be readily identified. The vegetationtype can be verified by pollen analysis. A total of 5meters of peat soil accumulated in Sweden in9,000 years or at a rate equal to 1 centimeter every 18 years. This is illustrated in Figure 9.6.Properties and UseOrganic soils are composed mainly of plant andanimal residues and some microbial decom-position products. The soils are very light andhave low bulk density, high CEC, and a high nitro-gen, phosphorus, and sulfur content.The absenceor low content of sand and silt particles, com-posed of minerals containing essential nutrients, FIGURE 9.6 Stratification found in a peat soil inmay result in low amounts of available potassium. Sweden, where changes in climate produced changes i n the kind of vegetation that formed the peat. (Data Organic soils must be drained before they can from Lucas, 1982.)be used for crop production. The upper soil layersthen become aerobic and the organic matter be-gins to decompose. This converts undecomposedpeat into well-decomposed organic soil called Archaeological Interestmuck. In time, the entire soil may disappear. This The excellent preservation ability in a peat bog isis a serious problem in a warm climate with a illustrated by examination of the famous Tollundrapid decomposition rate, as in the Florida Ever- man, who was found in 1950 in a peat bog inglades, where sugarcane is a major crop, together Denmark. The facial expression at death andwith a wide variety of vegetable crops (see Figure bristles of the beard were well perserved. Excel-9.7). Organic soils typically occupy the lowest lent fingerprints were made, and an autopsyelevation in the landscape, and crops grown in revealed the contents of the last meal, which con-such soils are the most likely to be killed by frost. sisted mainly of seeds, many of them weed seeds.Their low bulk density makes them susceptible to When found, the Tollund man was buried under 2wind erosion and desirable for sod production meters of peat that was estimated to have formed (see Figure 9.8). in the 2,000 years since his burial.
THE EQUILIBRIUUM CONCEPT 14 1 FIGURE 9.7 The subsidence rates for organic soils depends on the depth to the water table; the lower the water table the greater is the soil aeration and decomposition of organic matter. Subsidence is greater in Florida than in Indiana because of higher temperatures in Florida.THE EQUILIBRIUUM CONCEPT A Case ExampleDuring the early phase of soil formation, soil or- The pool, or amount of organic matter in a soil,ganic matter content increases. After a few centu- can be compared to a lake. Changes in the waterries, the soil attains a dynamic equilibrium con- l evel in a lake depend on the difference betweentent of organic matter; additions balance losses. the amount of water entering and leaving it. ThisEven though the organic matter content remains idea, applied to soil organic matter, is illustratedconstant from year to year, there is a significant at the top of Figure 9.9, showing that the change inturnover of organic matter and recycling of nu- soil organic matter is the difference between thetrients. amount of organic matter added and the amount that is decomposed.FIGURE 9.8 Organic soil landscape. Crop on left is Soil organic matter, labile and stable con-grass sod used for landscaping and in the rear is a sidered together, decomposes in mineral soils atwindbreak of willow trees. The lightness of the soil a rate equal to about I to 3 percent per year.makes the soil desirable for sod production; Assuming a 2 percent decomposition rate, andhowever, it also contributes to susceptibilty to winderosion. 40,000 kilograms in the plow layer of a hectare, 800 kilograms of soil organic matter would be decomposed or lost each year. Conversely, if 800 kilograms of humus or stable organic matter were formed from decomposition of residues added to the soil, the organic matter content of the soil would remain the same from one year to the next. The soil would be at the equilibrium level illus- trated in Figure 9.9. Decomposition of 800 kilograms of organic matter containing 5 percent nitrogen would result i n mineralization of 40 kilograms per hectare of nitrogen. On an acre basis, this would amount to about 40 pounds of available nitrogen per acre.
142 SOIL ORGANIC MATTER FIGURE 9.9 Schematic illustration of the equilibrium concept of soil organic matter as applied to a hectare plow layer weighing 2 million kilograms and containing 2 percent organic matter. (Values are similar on a pounds per acre basis for a 2-million pound acre-furrow-slice.)Assuming that the ratio of N:P:S in the soil organic At the Missouri Agricultural Experiment Station,matter is 10:1:1, there would be 4 kilograms per as a result of 60 years of cultivation, the soil losthectare (4 lb/acre) of phosphorus and sulfur min- one third of its organic matter (see Figure 9.10).eralized. Other nutrients are also mineralized, but The organic matter content decreased 25 percenttheir availability is more closely related to the soil during the first 20 years, 7 percent during the nextmineral fraction and mineral weathering. 20 years, and only 3 percent during the third 20- year period. The organic matter content of the soilEffects of Cultivation was moving to a new equilibrium level. In general, cultivated soils today contain about one half asEven on nonerosive land that is converted from much organic matter as they did before culti-grassland or forest to cultivated land, a rapid loss vation. Theoretically, if a soil is fallowed, so thatof organic matter resulting from cropping occurs. no plants grow and decomposition continues, the
THE EQUILIBRIUUM CONCEPT 143 the SOM content. In this case, converting land to agricultural use results in an equilibrium level that is higher than that of the virgin land. Maintenance of Organic Matter in Cultivated Fields The organic matter content of a soil is more diffi- cult to determine than the carbon content. As a result, changes in the soil carbon content are used as a measure of changes in organic matterFIGURE 9.10 Decline in soil nitrogen (or organic content. Multiplying the percent of soil carbon bymatter) with length of time of cultivation underaverage farming practices in Missouri. (Data from 1.72 is generally considered to equal the percentJenny, Missouri Agr. Exp. Sta. Bull. 324, 1933.) of organic matter. Soil organic matter is constantly subject to de- composition and loss. Therefore, each soil andorganic matter content would eventually ap- cropping situation, requires the addition of a cer-proach zero. On the other hand, if cultivated fields tain amount of organic matter each year to main-are allowed to revert to their original native vege- tain the status quo. In a 16-year experiment intation, the organic matter content will gradually Minnesota, 5 tons of crop residues per acre annu-i ncrease toward the equilibrium level that existed ally were required to maintain the original carbonprior to cultivation. content of the soil at 1.8 percent (see Figure 9.11). Soils in arid regions naturally have a low or- Addition of more than 5 tons per acre increasedganic matter content. Irrigating such soils and SOM content and lesser amounts resulted in aproducing crops that return a large amount of decline in the organic matter content. While landplant residues to the soil results in an increase in i s being farmed (excluding the desert land), it is FIGURE 9.11 Relationship between annual application rates of crop residues and content of organic carbon in soils after 11 years. (Data of Frye, Bennett, and Buntley, 1985).
144 SOIL ORGANIC MATTERvirtually impossible, to maintain the organic mat- kilograms per hectare would increase the soil car-ter content of the virgin soil. It is, therefore, bon content by about 800 kilograms per hectare.prudent to accept the organic matter content that Since World War II there have been large in-results from economical or profitable farming. creases in crop yields in many countries in spite In Figure 9.12, the effect of different yields of of claims of declining soil organic matter and soilcorn grain on soil carbon is given for a typical depletion. It appears that soil management sys-well-drained agricultural soil in southern Michi- tems that result in high crop yields maintain SOMgan. The grain was harvested and all of the re- at acceptable contents. Many soil managementmaining crop residues were returned to the soil, programs focus on the production of high cropincluding the roots that equal about 15 to 20 per- yields and proper crop residue managementcent of the total crop residues. Note that a corn rather than the organic matter content of soils, pergrain yield of 3,150 kilogram per hectare (kg/ha) se. One of the most serious causes of organic(50 bushels per acre) is predicted to maintain an matter loss is soil erosion that tends to prefer-equilibrium level equal to 1.0 percent organic car- entially remove the organic matter fraction. Highbon. By contrast, yields of 6,300 and 9,450 kilo- crop yields also mean more vegetative cover andgrams per hectare (100 and 150 bushels per acre) reduced soil erosion.are predicted to maintain equilibrium carbon lev- Effects of Green Manureels of 1.7 and 2.3 percent, respectively. Higheryields return more residues and maintain a higher Green manuring is the production of a crop for theorganic matter content, which in turn makes still purpose of soil improvement. These crops arehigher yields more likely. easily and inexpensively established and, fre- Another observation can be made from Figure quently, are grown during the fall, winter, and/or9.12. A yield of 6,300 kilograms per hectare would spring when the land would normally be un-result in about a 400 kilograms per hectare loss of protected by vegetation. Benefits include erosioncarbon if the soil contained 2.5 percent organic control and the accumulation of nutrients (intocarbon. Conversely, if the soil contained only 0.5 organic matter) that might otherwise be lost bypercent organic carbon, the same yield of 6,300 leaching. There is also less likelihood of pollution of water tables with nitrate nitrogen. In addition,FIGURE 9.12 Estimated annual soil-humus carbon organic matter from the green manure crop ischanges as related to corn (maize) yields and eventually incorporated into the soil, and the fol-organic carbon content of loamy soils in southern lowing crop benefits from the rapid release ofMichigan. (Adapted from Lucas and Vitosh, 1978.) nutrients from a large amount of readily decom- posable (labile) material. The humus that is formed contributes to the maintenance of the con- tent of soil organic matter. However, this is usu- ally a secondary reason why green manure crops are grown. HORTICULTURAL USES OF ORGANIC MATTER Organic materials used for horticultural purposes consist mainly of naturally occurring peat and moss and the products of composting. High-
HORTICULTURAL USES OF ORGANIC MATTER 14 5content organic matter materials are used as acomponent of soil mixes and as a mulch.Horticultural PeatsPeats that are used for soil amendments are gen-erally classified as moss peat, reed-sedge peat,and peat humus. Moss peat forms from moss veg-etation and reed-sedge peat from reeds, sedges,cattails, and other associated plants. Someproperties of common horticultural peats aregiven in Table 9.4. The low nitrogen content ofsphagnum peat means that it may not have FIGURE 9.13 Mulching a rose garden with peat moss. The mulch protects the soil surface from raindrop impact and maintains a highenough nitrogen to satisfy the needs of decom-posers and, thus, its use creates competition be- water-infiltration rate.tween decomposers and plants for any availablenitrogen that might be present in the soil mix. Thelow pH of sphagnum peat, however, makes it pings, or other plant and garbage wastes. Thesedesirable as a mulch for plants, such as azaleas materials can be composted with a resulting de-and rhododendrons, that grow well in acid soil. crease in the C:N ratio and production of an or- Peat moss makes a neat-looking surface that sets ganic material used for mulching and as a com- off plants and protects the soil from the disruptive ponent of soil mixes. Composting consists of i mpact of raindrops. The soil remains more po- storing or maintaining the organic materials in a rous and receptive to rain or irrigation water (see pile while providing favorable water content, aer- Figure 9.13). ation, and temperature. As the organic matter de- composes, much of the carbon, hydrogen, andComposts oxygen is released as carbon dioxide and water.Many gardners have organic residues with wide Nutrients, like nitrogen, are continuously reusedC : N ratios such as tree leaves, weeds, grass clip- and recycled by the microbes and conservedTABLE 9.4 Characteristics of Common Horticultural PeatsFrom Lucus, et al., Ext. Bull., 516, Micoigan State University, East Lansing, Micoigan.a Oven dry basis.
14 6 SOIL ORGANIC MATTERTABLE 9.5 Materials Recommended for microbial, and animal products. Even though it isMaking Compost a minor amount of the total soil organic matter, labile organic matter is very important in nutrient cycling. The stable organic matter is humus, which contributes to cation exchange capacity, water adsorption, and soil structure stability. Sta- ble organic matter decomposes very slowly, and as a result, releases (mineralizes) nutrients very slowly. Organic soils consist mainly of organic matter. They have light weight and are susceptible to wind erosion. Most organic soils require drainage for cropping, which aerates the soil and greatly i ncreases the rate of decomposition. The absence or very low content of minerals is associated with very low potassum supplying power. The organic matter content of a soil tends to approach an equilibrium. When the losses of or- ganic matter exceed additions, the soil organic matter content declines. Cultivated soils containFrom Kellogg, 1957. about half as much organic matter as they did before the soils were converted to cropping. Thewithin the composting pile. Thus, while there is a exception is desert soils where cropping may re-loss of carbon, oxygen, and hydrogen, other nutri- sult in an increase in organic matter content.ent elements are being concentrated. The occa- Horticultural peats and composts are organicsional turning of the compost pile increases aer- materials used for mulching and for soil mixes.ation and generally hastens rotting. The rottedmaterial has good physical properties and is agood source of nutrients. Small amounts of mate- REFERENCESrials, from the weeding of a garden, for example, Alexander, M. 1977. Soil Microbiology. 2nd ed. Johncan be composted in a black plastic bag. Wiley, New York. The low nitrogen content of many composting Allison, F. E. 1973. Soil Organic Matter and Its Role inmaterials may retard their decomposition. For this Crop Production. Elsevier, New York.reason, most composters add some nitrogen fer- Frye, W. W., 0. L. Bennett, and G. J. Buntley. 1985.tilizer to the compost pile. The nutritive value of "Restoration of Crop Productivity on Eroded or De-the compost can also be increased by the addition graded Soils." Soil Erosion and Crop Productivity. pp.of other materials. Some recommendations of 335-356. Am. Soc. Agronomy. Madison, Wis.the U.S. Department of Agriculture are given in Ta- Glob, P. V. 1954. "Lifelike Man Preserved 2,000 Years inble 9.5. Peat." Nat. Geog. Mag. 105:419-430. Jenny, Hans. 1933. "Soil Fertility Losses Under Missouri Conditions." Missouri Agr. Exp. Sta. Bull. 324. Colum- bia, Mo.SUMMARY Jenny, Hans. 1980. The Soil Resource. Springer-Verlag, New York.The total soil organic matter is composed of a Kellogg, C. E. 1957. "Home Gardens and Lawns". USDAlabile fraction and a stable fraction. The labile Agricultural Yearbook. pp. 665-688. Washington,fraction consists of readily decomposable plant, D.C.
REFERENCES 147Klemmedson, J. O. 1975. "Nitrogen and Carbon McDaniel, P. A. and L. C. Munn. 1985. "Effect of Tem- Regimes in an Ecosystem of Young Dense Pon- perature on Organic Carbon-texture Relationships in derosa Pine in Arizona." Forest Science. 21:163- Mollisols and Aridisols." Soil Sci. Soc. Am. J. 49: 168. 1486-1489.Lucas, R. E. 1982. "Organic Soil (Histosols)." Mi. Agr. Paul, E. A. 1984. "Dynamics of Organic Matter in Soils." Exp. Sta. Res. Report 435. East Lansing, Mi. Plant and Soil. 76:275-285.Lucas, R. E. and P. E. Rieke, and R. S. Farnham. 1971. Schreiner, O. and B. E. Brown. 1938. "Soil Nitrogen." "Peats for Soil Improvement and Soil Mixes." Mi. Agr. Soils and Men. USDA Yearbook of Agriculture, Wash- Ext. Bull. 516. East Lansing, Mi. ington, D.C.Lucas, R. E. and M. L. Vitosh. 1978. "Soil Organic Matter Van Veen, J. A. and E. A. Paul. "1981. Organic Carbon Dynamics." Mi. Agr. Exp. Sta. Res. Report 358. East Dynamics in Grassland Soils." Can. J. Soil Sci. Lansing, Mi. 61:185-210.
CHAPTER 10 SOIL MINERALOGYMinerals are natural inorganic compounds, with form to another, is essential to understandingdefinite physical and chemical properties, that are both the nature of the soils chemical propertiesso conspicuous in many granitic rocks. They are and the origin of its fertility. Since soils developbroadly grouped into primary and secondary min- from parent material composed of rocks and min-erals. Primary minerals have not been altered erals from the earths crust, attention is directedchemically since their crystallization from molten first to the chemical and mineralogical composi-l ava. Disintegration of rocks composed of primary tion of the earths crust.minerals (by physical and chemical weathering) CHEMICAL AND MINERALOGICALreleases the individual mineral particles. Many of COMPOSITION OF THE EARTHS CRUSTthese primary mineral particles become sand andsilt particles in parent materials and soils. Primaryminerals weather chemically (decompose) and About 100 chemical elements are known to existrelease their elements to the soil solution. Some in the earths crust. Considering the possible com-of the elements released in weathering react to binations of such a large number of elements, it isform secondary minerals. A secondary mineral not surprising that some 2,000 minerals haveresults from the decomposition of a primary min- been recognized. Relatively few elements anderal or from the precipitation of the decom- minerals, however, are of great importance inposition products of minerals. Secondary miner- soils. als originate when a few atoms react and Chemical Composition of the precipitate from solution to form a very small crys- Earths Crust tal that increases in size over time. Because of the generally small particle size of secondary miner- als, they dominate the clay fraction of soils. Approximately 98 percent of the mass of the Consideration of the characteristics of minerals earths crust is composed of eight chemical ele- found in soils, and their transformation from one ments (see Figure 10.1). In fact, two elements, 148
WEATHERING AND SOIL MINERALOGICAL COMPOSITION 149 are mined; in other areas, there is a deficiency of phosphorus for plant growth. Mineralogical Composition of Rocks Most elements of the earths crust have combined with one or more other elements to form the min- erals. The minerals generally exist in mixtures to form rocks, such as the igneous rocks, granite and basalt. The mineralogical composition of igneous rocks, and the sedimentary rocks, shale and sand- stone, are given in Table 10.1. Limestone is also an important sedimentary rock, which is com- posed largely of calcium and magnesium carbon- ates, with varying amounts of other minerals asFIGURE 10.1 The eight elements in the earths crust impurities. The dominant minerals in these rockscomprising over 1 percent by weight. The remainder are feldspars, amphiboles, pyroxenes, quartz,of elements make up 1.5 percent. (Data from Clark, mica, apatite, clay, iron oxides (goethite), and1924.) carbonate minerals.oxygen and silicon, compose 75 percent of it. WEATHERING AND SOILMany of the elements important in the growth of MINERALOGICAL COMPOSITIONplants and animals occur in very small quantities.Obviously, these elements and their compoundsare not evenly distributed throughout the earths The soil inherits from the parent material a min-surface. In some places, for example, phosphorus eral suite (a set of minerals) that typically con-minerals (apatite) are so concentrated that they tains both primary and secondary minerals. TABLE 10.1 Average Mineralogical Composition of Igneous and Sedimentary Rocks ° Present in small amounts.
15 0 SOIL MINERALOGYWeathering in soils results in the destruction of leached from the soil, particularly in humid re-existing minerals and the synthesis of new miner- gions.als. The minerals with the least resistance to The kaolinite, even though very resistant to fur-weathering disappear first. The minerals most re- ther weathering, may be decomposed and disap-sistant to weathering such as quartz, make up a pear from the soil. The reaction:greater proportion of the remaining soil and ap-pear to accumulate. As a consequence, the miner-alogy of a soil changes over time, when viewed ona time scale of hundreds of thousands of years. The decomposition of kaolinite resulted in the formation of gibbsite, a mineral more resistant to weathering than the kaolinite. Both kaolinite andWeathering Processes gibbsite are clay minerals that have distinctly dif-Numerous examples of weathering abound and ferent properties. Thus, as weathering proceedscan be observed every day. These include the and the mineralogical composition of the soilrusting of metal and the wearing down and disin- changes, the chemical and physical propertiestegration of brick walls. Mineral weathering is also change.stimulated by acid conditions. Respiration of The loss of silicic acid by leaching results in theroots and microorganisms produces carbon diox- progressive loss of silicon. There is a progressivei de that reacts with water to form carbonic acid i ncrease in the accumulation of aluminum, be- (H ² C0 3). An acid environment stimulates the re- cause the aluminum tends to be incorporated intoaction of water with minerals and is one of the resistant secondary minerals that accumulate inmost important weathering reactions. For exam- the soil. For this reason, an indicator of weather-ple, the reaction of a feldspar (albite) with water i ng intensity is the silica:alumina ratio (Si0 ² / (hydrolysis) and H + is as follows: A1 ² 0 3). As silicon is lost from the soil by leaching and aluminum accumulates, the silica:alumina ratio decreases. The Si0 ² /A1 ² 03 ratio decreases from 3:1 for albite to 1:1 for kaolinite. Other weathering reactions include oxidation, dissolution, hydration, reduction, and carbon-In the reaction a primary mineral, albite, was con- ation. An example of mineral weathering and the formation of clay minerals on the surface of averted to kaolinite. The first-formed kaolinite piece of basalt is shown in Figure 10.2.particles are very small. They increase in size asadditional ions attach or join the crystal, resultingi n an gradual increase in particle size. The clayfraction of soils tends to be dominated by parti- Summary Statementcles formed in this manner and particles like kao- 1. Mineral weathering is stimulated by an acidlinite are called clay minerals. Clay minerals tend soil environment.to be resistant to further weathering. Therefore, as 2. Plant-essential nutrient ions are released toprimary minerals, like albite, weather and disap- the soil solution and clay minerals (second-pear, clay minerals, like kaolinite, increase in ary) are formed.abundance. 3. Primary minerals disappear and secondary In the reaction, sodium was released as an ion, minerals, especially the clay minerals, accu-Na. In addition, some of the silicon in the albite mulate.was incorporated into kaolinite and some into 4. Much of the silicon released in weathering isH4 SiO4 (silicic acid). The silicic acid tends to be removed from the soil by leaching, unless
WEATHERING AND SOIL MINERALOGICAL COMPOSITION 151 prising about 47 percent, 28 percent, and 8 per- cent by weight, respectively (see Figure 10.1). Many soil minerals are silicate or aluminosilicate minerals. Oxygen is abundant on a weight basis, and because of the large size of the oxygen atom, oxygen occupies more than 90 percent of the vol- ume of the earths crust (and soil), as shown in Table 10.2. Note the generally small atomic size of the abundant cations listed in Table 10.2 as com- pared with the oxygen. In the minerals, the nega- tive charge of oxygen is balanced by the positive charge of cations. It is helpful to think of soil minerals as being composed of Ping-Pong balls with much smaller balls (like marbles), in the there is limited precipitation and leaching is i nterstices. The Ping-Pong balls represent large i neffective. The silica:alumina ratio de- negatively charged oxygen atoms, and the small creases as soils become more weathered. balls represent the positively charged cations in the interstices.Weathering Rate and Crystal Structure Silicon is a very abundant cation, which per- forms a role in the mineral world similar to theParticle size, through its effect on specific surface, role carbon plays in the organic world. The siliconis an important factor affecting the weathering ion fits in an interstice formed by four oxygenrate of minerals. That is, a given mineral in the silt i ons. The covalent bonding (electron sharing) be-fraction weathers faster than if the mineral is in tween oxygen and silicon forms a tetrahedron asthe sand fraction. Chemical bonding within the shown in Figure 10.3. The silicon-oxygen tetrahe-mineral crystal, however, is the major factor af- dron is the basic unit in silicate minerals. Thefecting weathering rate. valence of the silicon is +4 and that of the oxygen Oxygen, silicon, and aluminum are the threemost abundant elements in the earths crust, com- TABLE 10.2 Size, Percent Volume in Earths Crust, Coordination Number and Valence of the Most Abundant Elements in Soil Minerals
15 2 SOIL MINERALOGY The coordination number is the number of ions suits in the H+ of the water replacing magnesiumthat can be packed around a central ion. Four and iron ions from the crystal face, rather than theoxygen ions can be packed around a silicon ion, silicon. The weathering of olivine can be viewedresulting in a coordination number of 4 (as shown as the separation of the Si-O ²+, tetrahedra with the Mg2+i n Figure 10.3). Note that larger cations have a release of the and Fe as illustrated inlarger coordination number. Aluminum, iron, and Figure 10.4. The Si-O tetrahedra and the magne-magnesium ions are larger than silicon ions, and sium and iron ions encounter other ions in thealuminum, iron, and magnesium typically have a soil solution, where the ions can react and formcoordination number of 6. Potassium has a coor- new minerals. The magnesium and iron ions candination number of 8 and 12 (see Table 10.2). be absorbed by roots. Olivine weathers rapidly Different silicate minerals are formed, depend- and soils with a high content of olivine may have Mg²+ i ng on the way the silicon-oxygen tetrahedra are so much i n solution as to be detrimental to linked together and how the net charge of the some plants. Conversely, olivine is absent intetrahedra is neutralized. In olivine, the silicon- many soils because of its low resistance to weath- oxygen (Si-0) tetrahedra are bonded together by ering. magnesium and/or iron. The crystal is neutral and The net charge of individual Si-O tetrahedra has the formula of MgFeSi0 4 . The ratio of magne- may be partly or entirely neutralized by the com- sium and iron, however, is highly variable, result- mon sharing of oxygen between adjacent tetrahe- i ng in several olivine species. The model of oliv- dra. If adjacent Si-O tetrahedra share one oxygen, ine in Figure 10.4 shows that each magnesium single chains are formed as in pyroxene and au- ion, or iron ion, is surrounded by six oxygen ions; gite (see Figure 10.5). As shown in Figure 10.5, as three oxygen ions form each of the faces of two more sharing of oxygen by silicon occurs, miner- adjacent Si-0 tetrahedrons. Magnesium and iron als with double chains, sheets, and three- ions are larger than silicon and more oxygen ions dimensional structures are formed. Mica, and ka- are required to form an interstice large enough for olinite, have a sheet structure, and quartz has a them to occupy. The coordination number of three-dimensional structure. In the case of quartz magnesium and iron is 6. (Si0 ²) all of the charge is neutralized by the shar- The oxygen-silicon bonds are much stronger i ng of all of the oxygen with silicon. The result is athan the magnesium and iron bonds with oxygen. mineral with strong chemical bonding within theConsequently, reaction of olivine with water re- crystal and great weathering resistance. As more FIGURE 10.3 Models showing the tetrahedral (four-sided) arrangement of silicon and oxygen ions in silicate minerals. The silicon-oxygen tetrahedron on the left has the apical oxygen set off to one side to show the position of the silicon ion.
WEATHERING AND SOIL MINERALOGICAL COMPOSITION 153 FIGURE 10.4 Models representing the weathering of olivine. Olivine is composed of silicon-oxygen tetrahedra held together by iron and/or magnesium. Every other tetrahedron is "inverted," as shown by the light-colored tetrahedra in the olivine model on the left. During weathering, the silicon- oxygen tetrahedra separate with the release of iron and magnesium.oxygen sharing occurs, the oxygen-silicon ratio of weathering and soil formation). The weathering(ratio of oxygen ions to silicon ions) of the min- stages are based on the representative minerals ineral decreases. This is associated with increased the fine silt and clay fractions. Table 10.3 containsweathering resistance. 13 weathering stages grouped into minimal, mod- erate, and intensive weathering stages. Weathering stage I represents soils that haveMineralogical Composition Versus been subjected to little, if any, weathering andSoil Age leaching, because the soils contain weatherableThe difference in the weathering resistance of minerals such as gypsum, (CaSO 4 2H² 0), and ha-minerals has been used to develop a series of lite (NaCI). Each higher weathering stage is domi-weathering stages that parallel soil age (amount nated by minerals with greater weathering resis- FIGURE 10.5 Models showing the common arrangements of silicon- oxygen tetrahedra in silicate minerals and their relation to weathering resistance.
15 4 SOIL MINERALOGYTABLE 10.3 Representative Minerals and Soils Associated withWeathering Stagestance, and in effect, more developed or older erately weathered and they are quite fertile soils.soils. Soils in which the fine silt and clay fractions Most of the worlds corn and wheat are producedare dominated by minerals of the first five weath- on these soils.ering stages are considered to be minimally Soils of weathering stages 10 through 13 areweathered. These soils have sand and silt frac- considered to be intensively weathered. Mineralstions that are rich in primary minerals. The soils representative of the intensive weathering stagestend to occur where there has been a lack of time are secondary minerals with great weathering re-or water for weathering or where temperatures are sistance (see Table 10.3). These soils tend to oc-too low for effective weathering. Minimally weath- cur in warm and humid tropical regions whereered soils are generally fertile. there has been sufficient time to weather almost Soils dominated by minerals represented by all of the primary minerals . Many intensivelymoderate weathering are in stages 6 thru 9. These weathered soils have clay fractions dominated bysoils tend to have a significant amount of both kaolinite, gibbsite (Al(OH) 3 ), and iron oxides, in-primary minerals (quartz and muscovite mica) cluding hematite. The worlds oldest and mostand secondary minerals (vermiculite and weathered soils are high in content of oxides of smectite). Many temperate region soils are mod- aluminum (gibbsite), iron (hematite), and/or tita-
SOIL CLAY MINERALS 1RQenvironment of somewhat limited leaching. Crys- spaces between smectite particles contribute totallization of volcanic ash products in seawater hydraulic impermeability. Houston Black Clayoften produces montmorillonite. The expansion soils in Texas have a high smectite content andof layers permits easy diffusion of hydrated are characterized by contraction and develop-cations into the interlayer space and adsorption ment of wide cracks during the dry season and byon the planar surfaces. The dominant features of great expansion and closure of the cracks duringmontmorillonite are shown in Figure 10.12 and the wet season.some properties are given in Table 10.4. Kaolinite The individual clay layers in soils tend to occuras clusters that are much like a stack of playingcards. The more expansive the clay, the more Reference to the formation of kaolinite was madelikely layers will separate, causing the clay to exist i n regard to the weathering of albite feldspar. Inas very small-sized particles. As a consequence, that case, kaolinite formation was by crystalliza-smectites occur as very small particles, which tion from solution. Another mechanism for kao-give this type of clay great capacity for swelling linite formation is the partial disintegration of 2:1and shrinking with wetting and drying. The small clays. The stripping away of one Si-0 sheet, from FIGURE 10.13 Model of kaolinite showing two offset layers. The upper layer surfaces are composed of hydroxyls (light-colored). These hydroxyl are bonded by the hydrogen of the hydroxyl to the oxygen of adjacent layers, producing a nonexpanding clay. Cation exchange capacity is low and originates at the edges due to deprotonation of exposed hydroxyls. A hydrated cation is adsorbed to such a cation exchange site.
16 0 SOIL MINERALOGY a 2:1 layer, converts the 2:1 layer into a 1:1 layer, volcanoes and are deposited on land masses be- typical of kaolinite. This mode of formation is fore the atoms become organized into crystals. In favored by an acid weathering environment that is the humid tropics, amorphous minerals weather found in older soils or intensively weathered soils rapidly into an amorphous clay mineral calledwhere silicon is being removed by leaching. allophane. Soils high in allophane have a gel-like The 1:1 structure has layers that have a plane of consistency when wet and lack the grittiness as- oxygens on one side and a plane of hydroxyls on sociated with sand particles. There is an enor- the other. Stacking of the layers results in a strong mous surface area and great reaction with, or attraction between oxygen of one layer and the adsorption of, organic matter, giving the organic hydrogen of hydroxyl (hydrogen bonding) of an matter protection against decomposition. Charac- adjacent layer, resulting in a nonexpanding clay. teristic features of soils high in allophane include Kaolinite particles tend to be large in size (many a high content of organic matter, high water- are silt sized), and to have great resistance to holding capacity, high and variable cation ex- further weathering. The tetrahedral and octahe- change capacity, and low load-bearing strength. dral sheets may retain some isomorphous substi- Slowly, allophane becomes more organized tution that can contribute to cation exchange ca- (more crystalline) and becomes halloysite-a pacity. This appears to be very minor and the mineral with structure and composition similar to cation exchange capacity is low, relative to the kaolinite. A weathering sequence from volcanic other clay minerals discussed thus far (see Table ash weathering is allophane-halloysite-kaolinite. 10.4). The cation exchange capacity occurs Soils high in allophane are important in the moun- mainly because of deprotonation of exposed hy- tain chain that extends from southern South droxyl at the edges. A model showing some America to Alaska and, also, in Hawaii. properties of kaolinite is given in Figure 10.13. Oxidic Clays The large size of kaolinite particles often givessoils high in kaolinite clay content good physicalproperties. Many of the kaolinite particles occur Kaolinite formation from 2:1 minerals is indicativei n the coarse clay (0.0002 to 0.002 mm diameter) of a leaching regime that is very effective in re-and silt fractions. Smectite particles typically oc- moving silicon. Under such conditions, kaolinitecur in the fine clay (less than 0.0002 mm diame- may slowly weather, resulting in the decom-ter). Two Bt horizons may contain the same position of the layer sheets into their componentamount of clay but, if the clay is kaolinite, the parts. The Si-O tetrahedra are transformed intohorizon could be quite water permeable com- Si-OH tetrahedra, H4SiO4 , (silicic acid), that ispared with a Bt horizon containing smectite. soluble and removed by leaching. Aluminum-OHSmectite is a common clay of U.S. midwestern octahedra are released and slowly polymerize toand Great Plains soils, in United States, and a form an aluminum hydrous oxide (Al(OH) 3),significant number of the soils have very slowly or called gibbsite. Gibbsite particles tend to be platyi mpermeable Bt horizons. By contrast, kaolinite is and hexagonal in shape like the sheet shown incommon in soils on the coastal plain of south- Figure 10.7. Many layers stack on top of each othereastern United States where soils are much older. and exist as clay particles in soils. Gibbsite is veryEven so, impermeable claypan Bt horizons are not i nsoluble and resistant to further weathering andcommon on the southeastern coastal plain. is representative of weathering stage 11 (see Ta- ble 10.3). The mineral is common in intensivelyAllophane and Halloysite weathered soils that also contain abundant iron oxides, such as hematite (Fe ² 03) and goethiteNoncrystalline (amorphous) minerals exist where (FeOOH). Only rare and unusual soils have a sig-glassy ash and cinders are thrown into the air by nificant amount of titanium oxide (anatase),
ION EXCHANGE SYSTEMS OF SOIL CLAYS 16 1which is representative of weathering stage 13. ogy of the clay fraction is dominated by oxidicFew parent materials contain enough titanium for clays, together with kaolinite.a significant amount of anatase to accumulate via 5. Volcanic activity produces amorphous ashweathering and soil formation. and cinders that weather to form amorphous The oxide clays are abundant in many red- allophane. Allophane slowly crystallizes tocolored soils in the humid tropics. These soils are form kaolinite.typically acid and, in an acidic environment, 6. Over time, weathering produces changes insome of the exposed hydroxyls on particle sur- the mineralogical composition of soils,faces of oxidic clays adsorb a H + (protonate) which is reflected in the soils physical andand form sites of positive charge (Al-OH + H + = chemical properties. The cation exchange ca-Al-OH² +). The kaolinite clay has a net negative pacity, for example, increases to a peak withcharge, which promotes attraction of the kaolinite weathering in soils dominated by 2:1 claysand oxidic clays and formation of extremely sta- (moderately weathered), which is followedble aggregates or peds. The peds, which are by a very low cation exchange capacity indense and frequently the size of sand grains, are soils dominated by oxidic clays (intensivelyvery resistant to deformation and give the soil weathered).physical characteristics similar to soils domi-nated by quartz sand; that is, soils with an abun-dance of oxidic clays have high water-infiltration I ON EXCHANGE SYSTEMS OFrates and resist erosion. The soil can be plowed SOIL CLAYSshortly after a rain. A small amount of available Some soils are dominated by layer silicate clays,water is retained at field capacity. The water isheld at low tension or high potential, and the some by oxidic clays, and some soils have bothwater is easily and rapidly absorbed by roots. kinds of clay in abundance. This results in threeShort periods of water stress for crops may occur unique ion-exchange systems in mineral soils:even though the soils are in the tropical rain forest (1) layer silicate, (2) oxide, and (3) oxide-coated, layer silicate. The type and kind of ion exchangezone. have important consequences for plant growthSummary Statement and soil management. Layer Silicate System1. There is a kinship amoung the crystalline alu- minum silicate clay minerals because all con- Layer silicate system soils are overwhelmingly sist of Si-0 tetrahedra sheets and Al-OH octa- negatively charged with high cation exchange ca- hedra sheets. pacity (CEC). The cation exchange capacity origi-2. The major differences in the aluminum sili- nates mainly from isomorphous substitution. cate clays result from the ratio of tetrahedra These negatively charged sites constitute perma- and octahedra sheets and the nature and nent charge that is not affected by pH. The charge amount of isomorphous substitution. A sum- may be modified, however, to a small extent by mary of the major features of these clays is minor or thin coatings of oxidic clays on the parti- given in Table 10.4. cle surfaces. Some cation exchange capacity orig-3. The 2:1 clays are transformed into 1:1 clay by i nates from the dissociation of hydrogen (depro- the loss of a Si-O tetrahedral sheet. This is one tonation) from edge AI-OH groups that is favored formation mode of kaolinite. by high pH (high OH concentration):4. Near the end of the weathering sequence, gibbsite formation becomes important. In the [ clay edge]AI-OH + OH - oldest and most weathered soils, the mineral- = [clay edgelAl-0 - + H ² O
REFERENCES 163diate between the other two systems. Many of the Ultimately, soils become "weathered out" andred-colored tropical soils have an ion exchange have almost no capacity to support plants. Suchsystem that is greatly affected by both silicate and soils may be mined for ore. Clay edge Al-OHoxidic clay. protonate in acid soils and form sites of an- ion exchange capacity and deprotonate in alka- line soils and form sites of cation exchange ca- pacity.SUMMARYIn an acid soil environment, weathering of miner-als is stimulated. Minerals weather at different REFERENCESrates, depending on their resistance to weath- Birkeland, P. W. 1974. Pedology, Weathering, and Geo-ering. morphological Research. Oxford, New York. In general, primary minerals weather and sec- Soil Chemistry, 2nd ed. John Wiley, New York. Bohn. H. L., B. L. McNeal, and G. A. OConnor. 1985.ondary minerals are formed. Over time, the miner-alogical composition of the soil changes and, Clarke, F. W. 1924. "Data of Geochemistry." U.S. Geo.consequently, soil properties change. Survey Bull. 770. Washington, D.C. Weathering releases ions to the soil solution. Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. JohnMany of these ions are plant nutrients. Some ions Wiley, New York.precipitate to form secondary minerals and some Jackson, M. L. and G. D. Sherman. 1953. "Chemicali ons are leached from the soil. Weathering of Minerals in Soils." Advances in Minimally and moderately weathered soils tend Agronomy. 5:219-318.to be rich in primary minerals and to be fertile Jenny, H. 1980. The Soil Resource. Springer-Verlag,with high cation exchange capacity. Intensively New York. Soil Sci. Soc. Am. 1977. Minerals in the Soil Environ-weathered soils are depleted of primary minerals. ments. Soil Sci. Soc. Am. Madison, Wis. Few nutrient ions are released and soils are infer- Uehara, G. and G. Gillman. 1981. Mineralogy, Chem-tile. The clay minerals in intensively weathered istry, and Physics of Tropical Soils with Variable soils have low cation exchange capacity. Charge. Westview, Boulder. Nutrient ions adsorbed to the surfaces of clay White, R. E. 1987. Introduction to the Principles andparticles represent a reservoir of nutrients that is Practice of Soil Science. 2nd ed. Blackwell Sci. Pub. exchangeable and available to plants. London.
CHAPTER 11 SOIL CHEMISTRYA total chemical analysis of a soil may reveal little age chemical composition of igneous rocks isthat is useful in predicting the ability of the soil to similar to the mineralogical composition of manysupply plant nutrients. Instead, it is usually a rela- minimally and moderately weathered soils. Suchtively small part of the total amount of an element soils contain much quartz, feldspar, and mica inthat is available for plant growth. For these rea- the sand and silt fractions, 2:1 layer silicate clayssons, the discussion of soil chemistry focuses on i n the clay fraction, and much of the negativei on exchange reactions, solubilities, and mineral charge of the clays neutralized by the adsorptionand biochemical transformations. These reac- of calcium, magnesium, sodium, and potassiumtions are illustrated in Figure 11.1. ions. When soils weather and the mineralogical composition changes over time, there is a corre- sponding change in chemical composition. Dur-CHEMICAL COMPOSITION OF SOILS i ng soil formation, there is a preferential loss of silicon relative to aluminum and iron. This is re-The average chemical composition of igneous flected in the chemical analysis of the intensivelyrocks is given in Table 11.1. On an oxide basis, weathered Columbiana clay soil in Costa Ricasilicon and aluminum are first and second in (see Table 11.1). The soil contains only 26 per-abundance. This reflects the dominance of sili- cent silica, and the sum of iron and alumumium iscate and aluminosilicate minerals in igneous 69 percent (on an oxide basis). The silica:rocks. Iron is next in abundance in igneous rocks. alumina ratio decreased from 3.75 to 0.53 as aNext most abundant, are four cations that are result of weathering and soil formation. Thei mportant in balancing the charge of silicon- chemical composition of the Columbiana soil re-oxygen tetrahedra in minerals-calcium, magne- flects a high oxidic clay content. Release and losssium, sodium, and potassium. The remaining 1 or by leaching of calcium, magnesium, sodium, and2 percent includes the other elements. The aver- potassium were more rapid than silica, and this is 164
I ON EXCHANGE 165 FIGURE 11.1 Major chemical processes and reactions in soils. Weathering of minerals and decomposition of organic matter supply ions to the ion pool of the soil solution. Ions are lost from the pool by plant uptake and leaching. Some i ons precipitate and some ions form secondary minerals, especially clay minerals, and are subject to dissolution. Ions also sorb onto and desorb from cation and anion exchange sites.reflected in the low content of these four cations tively and postively charged surfaces, respec-i n the intensively weathered soil relative to igne- tively. Such ions are readily replaced or ex-ous rocks. changed by other ions in the soil solution of similar charge, and thus, are described by the term, ion exchange. Of the two, cation exchangeI ON EXCHANGE is of greater abundance in soils than anion ex- change.Ion exchange is of great importance in soils (see Nature of Cation ExchangeFigure 11.1). Ion exchange involves cations andanions that are adsorbed from solution onto nega- Cation exchange is the interchange between a cation in solution and another cation on the sur-TABLE 11.1 Chemical Composition of Average face of any negatively charged material, such asIgneous Rocks and an Intensively Weathered Soil clay colloid or organic colloid. The negative charge or cation exchange capacity of most soils is dominated by the secondary clay minerals and organic matter (especially the stable or humus portion of SOM). Therefore, cation exchange re- actions in soils occur mainly near the surface of clay and humus particles, called micelles. Each micelle may have thousands of negative charges that are neutralized by the adsorbed or exchange- able cations. Assume for purposes of illustration - that X represents a negatively charged ex- changer that has adsorbed a sodium ion (Na), producing NaX. When placed in a solution con- taining KCI, the following cation exchange reac- tion occurs:Adapted from Bohn, McNeal, and OConnor, 1985.
166 SOIL CHEMISTRYI n the reaction, K+ in solution replaced or ex-changed for adsorbed (exchangeable) Na, re-sulting in putting the adsorbed Na in solutionand leaving K + adsorbed as KX. Cations are ad-sorbed and exchanged on a chemically equiva-lent basis, that is, one K+ replaces one Na, andtwo K+ are required to replace or exchange for t+one Ca . The negatively charged micellar surfaces form aboundary along which the negative charge is lo-calized. The cations concentrate near this bound- from clay surfaceary and neutralize the negative charge of the Distance FIGURE 11.2 Distribution of cations and anions inmicelle. The exchangeable cations are hydratedand drag along the hydration water molecules as the vicinity of a negatively charged surface.they constantly move and oscillate around nega-tively charged sites. The concentration of cations Cation Exchange Capacity of Soilsis greatest near the micellar surfaces, where thenegative charge is the strongest. The chargestrength decreases rapidly with increasing dis- The cation exchange capacity of soils (CEC) istance away from the micelle, and this is associ- defined as the sum of positive (+) charges of theated with a reduction in cation concentration adsorbed cations that a soil can adsorb at a spe-away from the micelle. Conversely, the negatively cific pH. Each adsorbed K+ contributes one + 2+charged micellar surface repels anions. This re- charge, and each adsorbed Ca contributessults in a decreasing concentration of cations and two + charges to the CEC. The CEC is the sum ofan increasing concentration of anions with dis- the + charges of all of the adsorbed cations.tance away from the micellar surface. At some (Conversely, the CEC is equivalent to the sum of distance from the micellar surface, the concentra- the - charges of the cation exchange sites.) tion of cations and anions is equal. These features The CEC is commonly expressed as centimoles are illustrated in Figure 11.2. of positive charge per kilogram [cmol(+)/kg], An equilibrium tends to be established between also written as cmol/kg, of oven dry soil. A mole 23 the number of cations adsorbed and the number of positive charge (+) is equal to 6.02 x 10 of cations in solution. The number of cations in charges. For each cmol of CEC per kg there are solution is much smaller than the number ad- 6.02 x 1021 + charges on the adsorbed cations. sorbed (generally 1 percent or less) unless the For many years the CEC of soils was expressed content of soluble salts is high. Roots absorb as milliequivalents of cations adsorbed per 100 cations from the soil solution and upset the equi- grams of oven dry soil (meq/ 100 g). Many agricul- librium. The uptake of a cation is accompanied by tural publications still use meq/100 g, which the excretion of H+ from the root and this restores means that many persons need to be familiar with the charge equilibrium in both the plant and soil. both systems. The CEC values are the same in Excretion of hydrogen ions tends to increase the both systems; that is, a CEC of 10 cmol/kg is equal acidity in the rhizosphere and the H + may be to a CEC of 10 meq/100 g adsorbed on micelles. The H + is weakly adsorbed The CEC of a soil is equal to the CEC of both the onto clay (ionic bonding); however, it is strongly mineral and organic fractions. The major source adsorbed to carboxyl groups of humus (covalent of CEC in the mineral fraction comes from the bonding). clay. The negative charge of organic matter is due
I ON EXCHANGE 167mainly to dissociation of H+ (deprotonation)from the -OH of carboxyl and phenolic groups.The CEC of soil organic matter (SOM) ranges from100 to 400 cmol/kg, depending on degree of de-composition. There is an increase in the groupscontributing to the CEC as organic matter be-comes more decomposed or more humified. Asmentioned in the previous chapter, the clays havegreat variation in CEC, ranging from less than 10 cmol/kg for oxidic clay, to a 100 cmol/kg ormore for some 2:1 clay. The sand and silt fractionscontribute little to the CEC of soils. As a conse-quence, the CEC of soils is affected mainly by theamount and type of clay and the amount anddegree of decomposition of the organic matter. In the A horizons of mineral soils, the organicmatter and clay frequently make similar contribu-tions to the CEC. In subsoils, such as Bt horizonswhere clay has accumulated, the clay commonlycontributes more to the CEC than the organic mat-ter. The accumulation of humus in Bhs or Bh FIGURE 11.3horizons contributes much CEC to these subsoilhorizons. The cation exchange capacities in a sand-textured soil at the soils natural pH and determined at pH 7.0 and 8.2. The increases inCation Exchange Capacity versus cation exchange capacity with increasing pH areSoil pH closely related to the content of organic carbon (organic matter) in this soil which has a Bh horizonThe definition of CEC specifies that the CEC ap- and contains very little clay. (Adapted from Holzhey,plies to a specific pH because the CEC is pH Daniels, and Gamble, 1975).dependent. To make valid comparisons of CECbetween soils and various materials, it is neces-sary to make the determination of CEC at a com-mon pH. The CEC is positively correlated with pH; i mportant in the management of intensivelytherefore, acid soils have a CEC less than the weathered and acid soils in tropical regions be-maximum potential CEC. cause of the generally low CEC and the highly Most of the CEC of 2:1 clays is permanent or pH-dependent nature of the CEC. For minimallynon-pH dependent. By contrast, the CEC of SOM is and moderately weathered mineral soils, whereentirely pH-dependent. Soils high in organic mat- much of the CEC originates from the permanentter content, kaolinite (1:1 clay), and oxidic clays charge of 2:1 clays, changes in CEC resulting fromhave relatively large changes in CEC with changes changes in soil pH have much less importance.i n pH owing to the predominance of pH- For plants, the current soil pH is the relevant pH.dependent charge. The changes in the CEC with Values given for the CEC are for soils at theirchanges in pH of the horizons in a very sandy soil natural, or current pH, unless otherwise noted.that has a Bh horizon is shown in Figure 11.3. The CEC at the soils current pH is called the Changes in CEC with changes in soil pH are effective cation exchange capacity (ECEC).
168 SOIL CHEMISTRYKinds and Amounts ofExchangeable CationsThe major sources of cations are mineral weather-i ng, mineralization of organic matter, and soilamendments, particularly lime and fertilizers. Theamounts and kinds of cations adsorbed are theresult of the interaction of the concentration ofcations in solution and the energy of adsorption ofthe cations for the exchange surface. The cationscompete for adsorption. As the concentration of acation in the soil solution increases, there is ani ncreased chance for adsorption. The morestrongly a cation is attracted to the exchange sur-face (energy of adsorption); the greater is thechance for adsorption. The energy of adsorption of specific ions isrelated to valence and degree of hydration. The cules are attracted to ions, the greater is the num-energy of adsorption of divalent cations is about ber of hydrated molecules and the hydrated ra-twice that of monovalent cations. For cations of dius of the ions.equal valence, the cation with the smallest hy- The differences in the hydrated radius and va-drated radius is most strongly adsorbed because l ence of Ca t+ and Na are illustrated in Figureit can move closer to the site of charge. The dehy- 11.4. Calcium is adsorbed more strongly than so-drated and hydrated radi of four monovalent dium because: (1) it is divalent, and (2) it has thecations are given in Table 11.2. Rubidium (Rb) is smallest hydrated radius. As a result of the smallthe most strongly adsorbed, whereas lithium (Li) energy of adsorption of sodium, it is more likely tois the most weakly adsorbed. Note that the hy- exist in the soil solution and be removed from thedrated radius is inversely related to the nonhy- soil by leaching. As a result of the strong energy ofdrated radius. This is because the water mole- adsorption, calcium is typically more abundant ascules can move closer to the charge that is located an exchangeable cation than are magnesium, po-at the center of a small ion and, therefore, the tassium, or sodium. The energy of adsorption se-water molecules are more strongly attracted to quence is: Ca >Mg > K > Na. The cmol/kgsmaller ions. The more strongly that water mole- (meq/ 100 g) of these four cations for a moderately weathered soil is given in Table 11.3. Note that their abundance follows the energy of adsorption sequence and the dominance of calcium.TABLE 11.2 Ionic Radii and Exchange Efficiency of The symbol, X, has already been used in aSeveral Monovalent Ions cation exchange reaction, as in NaX. In NaX, a Na is adsorbed onto a cation exchange site. By transposing the X to form XNa, exchangeable so- dium is formed. XNa is an abbreviation for exchangeable Na. Pronounce XNa, as "X Na," or as exchangeable Na, and write it as XNa. Ex- changeable calcium is XCa. This nomenclature will be used in reference to the exchangeable cations.
I ON EXCHANGE 169TABLE 11.3 Exchangeable Calcium, Magnesium, data in Table 11.3. A plow layer would containPotassium and Sodium in Tama Silt Loam enough XCa to last a high-yielding corn crop 112 years, the XMg would last for 27 years, whereas the XK would provide only a 2-year supply. When the exchangeable cations in the underly- i ng horizons are considered, it can be seen that the exchangeable cations are an important supply of available nutrients. Small, but important, quan- tities of other cations, including iron, manganese, copper, and zinc also exist in the soil as ex- changeable cations. Anion ExchangeAdapted from Soil Survey Investigations Report, No. 3, I owa,SCS, USDA, 1966. Anion exchange sites arise from the protonationExchangeable Cations as a Source of of hydroxyls on the edges of silicate clays and onPlant Nutrients the surfaces of oxidic clays. The anion exchange capacity is inversely related to soil pH and is,Of the four cations referred to in Table 11.3, three perhaps, of greatest importance in acid soils dom-are essential for plants. The sodium may be ab- i nated by oxidic clays. Nitrate (N0 3 - ) is verysorbed by plants, may substitute for some po- weakly adsorbed in soils and, as a consequence,tassium, but sodium is not considered an essen- remains in the soil solution where it is very sus-tial element. It is, however, needed by animals, ceptible to leaching and removal from soils.and its uptake by plants and its presence in food is Anion exchange reactions with phosphate mayi mportant. It is paradoxical that, in regard to XCa result in such strong adsorption as to render theand XK, the XCa is much more abundant in soils phosphate unavailable to plants. Uptake of anionsthan is XK. On the other hand, plants generally by roots is accompanied by the excretion of OH -absorb much more potassium than calcium. The or HC03 - .result is that calcium is rarely deficient in soils for In general, plants absorb as many anions asplant growth; by contrast, potassium frequently cations. The availability to plants of the anionsli mits crop yields in the humid regions and is a nitrate, phosphate, and sulfate is related to miner-common element in fertilizers. This is reflected in alization from organic matter, as well as anionthe data of Table 11.4, which are based on the exchange. TABLE 11.4 Amounts of Exchangeable Calcium, Magnesium and Potassium in 2 Million Pounds in the Ap horizon of Tama Silt Loam and Adequacy for Plant Growth
17 0 SOIL CHEMISTRYSOIL pH herit these properties from the parent material. Throughout the world there are parent materialsThe pH is the negative logarithm of the hydrogen and soils that contain several percent or more ofion activity. The pH of a soil is one of the most calcium carbonate; they are calcareous. Wheni mportant properties involved in plant growth. calcareous soil is treated with dilute HCI, carbonThere are many soil pH relationships, including dioxide gas is produced, and the soil is said to effervesce.those of ion exchange capacity and nutrient avail-ability. For example, iron compounds decrease insolubility with increasing pH, resulting in manyi nstances where a high soil pH (soil alkalinity) Carbonate Hydrolysis The hydrolysis of cal-causes iron deficiency for plant growth. cium carbonate produces OH - , which contrib-Determination of Soil pH utes to alkalinity in soils:Soil pH is commonly determined by: (1) mixingone part of soil with two parts of distilled water or Calcium carbonate is only slightly soluble, andneutral salt solution, (2) occasional mixing over a this reaction can produce a soil pH as high as 8.3,period of 30 minutes to allow soil and water. to assuming equilibrium with atmospheric carbonapproach an equilibrium condition, and, (3) mea- dioxide. In calcareous soil, carbonate hydrolysissuring the pH of the soil-water suspension using a controls soil pH. When a soil contains Na 2 CO 3 ,pH meter. A rapid method using pH indicator dye the pH may be as high as 10 or more, which isis illustrated in Figure 11.5. The common range of caused by the greater solubility of Na 2 CO 3soil pH is 4 to 10. and greater production of OH - by hydrolysis inSources of Alkalinity a similar manner. Sodium-affected soils with 15 percent or more of the CEC saturated withParent materials have a wide ranging mineral- Na are called sodic and are also highly alka-ogical composition and pH, and young soils in- line. FIGURE 11.5 To determine soil pH with color indicator dye solution: (1) Place a small amount of soil in the folded crease of a strip of wax paper; (2) add 1 or 2 drops more i ndicator than the soil can absorb, tilt up and down to allow indicator to equilibrate with soil, and (3) separate excess dye from soil with tip of a pencil and match dye color with the color chart.
SOIL pH 17 1 Mineral Weathering Some rocks and minerals Sources of Acidity weather to produce an acidic effect. The weather- Three important processes, or reactions, contrib- i ng of many primary minerals, however, contrib- ute to a more or less continuous addition of H + to utes to alkalinity. This is the result of the con- soils, and these processes tend to make soils sumption of H + and the production of OH - . For acid. In all soils respiration by roots and other soil example, the hydrolysis of anorthite, (calcium organisms produces carbon dioxide that reacts feldspar), produces a moderately strong base: with water to form carbonic acid (H2CO3). This is a weak acid, which contributes H+ to the soil solution. Second, acidity is produced when or- ganic matter is mineralized, because organic acids are formed and the mineralized nitrogen and sulfur are oxidized to nitric and sulfuric acid, I n the reaction, an aluminosilicate clay mineral respectively. Third, natural or normal precipi- was formed together with the moderately strong tation reacts with carbon dioxide of the atmo- base, calcium hydroxide. The net effect is basic. sphere and the carbonic acid formed gives natural The generalized weathering reaction, in which M precipitation a pH of about 5.6. The results of these reactions add acidity continuously to soils. represents metal ions such as calcium, magne- In desert regions there is little precipitation sium, potassium, or sodium, is and, thus, these reactions contribute little acidic i nput to soils. As precipitation increases there is an increase in all three of the previously men- tioned processes. A larger amount of precipitation As long as a soil system remains calcareous, contains more acidity. Greater precipitation re- carbonate hydrolysis dominates the system and sults in more plant growth, causing more respira- maintains a pH that ranges from 7.5 to 8.3 or tion and organic matter mineralization. Many more. Mineral weathering is minor under these studies have shown a relationship between an- conditions, and the mineralogy and pH of the nual precipitation, the leaching of carbonates, soils change little, if at all, with time. The develop- and soil pH, as given in Figure 11.6. ment of soil acidity requires the removal of the In soils with a pH of 7, there is a balance in the carbonates by leaching. When acidity develops, processes or reactions that produce alkalinity and the weathering of primary minerals is greatly in- acidity. On the central U.S. Great Plains this oc- creased, causing an increase in the release of curs with about 26 inches (65 cm) of annual pre- cations-calcium, magnesium, potassium and cipitation (near the Iowa-Nebraska border). The sodium. Leaching of these cations in humid re- dominant upland soils have developed under a gions, however, eventually results in the develop- grass vegetation and are quite fertile for agricul- ment of soil acidity. tural crops. Calcium and magnesium are alkaline earth metals or cations, and potassium and sodium are alkali metals. These cations are called basic Development and Properties of cations because soils tend to be alkaline (basic) Acid Soils or neutral when the CEC is entirely saturated with I n a calcareous soil, OH - from carbonate hydroly- them. Leaching of the basic cations (Ca, Mg, K, sis reacts with any H + that is produced to form and Na) results in their replacement by Al" and water; thus, calcareous soils remain alkaline in and H+ are called the acidic cations. H+, and soils then become acid. The cations A13 + spite of the near constant production or addition of H + . As long as soils remain calcareous, they
17 2 SOIL CHEMISTRY FIGURE 11.6 General relationship of upland soils between annual precipitation, depth of leaching of carbonates, and pH of the surface soil in the central United States. (Adapted from Jenny, 1941.)remain alkaline. Development of neutrality, and leached soils, all of the horizons are acid. Byeventually acidity, is dependent on the removal of contrast, many moderately weathered andthe carbonates. This requires sufficient precipi- leached soils have acid A and B horizons andtation to cause effective leaching. This amount of nuetral or alkaline (many being calcareous) Cprecipitation also results in considerable acidic horizons.i nput. The weathering of primary minerals and therelease of basic cations tend to repress the acidi-fying effects of respiration, precipitation, and min- Role of Aluminum After leaching has removederalization of organic matter. Any H + i n the soil the carbonates, a progressive loss of XCa, XMg,solution neutralizes the OH - produced by min- XK, and XNa occurs as leaching continues. Foreral weathering and shifts the weathering reac- example, the removal of an exchangeable po-tions to the right, or encourages further weather- tassium ion, K+, is accompanied by the removali ng of primary minerals and formation of OH - . of an acid anion, like nitrate. The H + associatedDespite the neutralizing effect of mineral weather- with the nitrate remains behind and competes fori ng, soils in humid regions become increasingly adsorption onto the exchange position vacated byacid with time as leaching continues. the potassium. Adsorption of H+ at the edge of The effects of the acidifying processes are pro- silicate clay minerals makes aluminum unstable, Subsequent hydrolysis of the released A1 3 + pro-gressive; the effects producing acid A horizons and it exits from the edge of the clay lattice.first, then acid B horizons, and, lastly, acid Chorizons. Many minimally weathered and leached duces H + :soils have a neutral or alkaline reaction in allhorizons and in intensively weathered and
SOIL pH 17 3 Hydroxy-aluminum from the preceding reaction exchangeable cation (XAI). The aluminum ion hydrolyzes to produce additional H + : has a valence of 3 + , and it is adsorbed much more strongly than divalent and monovalent cations. The amount of XAI increases over time as the soil becomes more leached and more acid. The in- Consequently, acidity in soil stimulates the devel- crease in XAL is accompanied by a decrease in opment of additional acidity through aluminum XCa, XMg, XK, and XNa. hydrolysis, and aluminum hydrolysis becomes a very important source of H+ when soils become acid. Moderately versus Intensively Weathered The hydroxy-aluminum (hydroxy-Al) formed in Soils As long as soils remain only moderately the preceding reactions consists of AI-0H octahe- weathered, the weathering of primary minerals dra that polymerize and produce large cations and the release of calcium, magnesium, po- with a high valence. These large cations are very tassium, and sodium contribute to the mainte- strongly adsorbed onto cation exchange sites be- nance of a good supply of these elements. These cause of their high valence. In fact, the adsorption elements, or basic cations, occupy most of the is so strong that they are not exchangeable in the cation exchange sites, and there is little ex- sense that XCa and XNa are exchangeable. Sites changeable aluminum. As moderately weathered that adsorb hydroxy-aluminum cease to func- soils evolve into intensively weathered soil, tion for cation exchange, and the adsorption of however, a considerable change in mineralogical hydroxy-aluminum reduces the CEC. This is one composition occurs. The high CEC 2:1 clays dis- more reason for the soils pH-dependent cation appear, and low CEC clays, such as kaolinite and exchange capacity. The adsorption of hydroxy- oxidic clay, accumulate. There is a large decrease aluminum in the interlayer space of expand