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
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.