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  1. 1. Chapter 12: SoilsMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
  2. 2. Soils• Soil and Regolith• Soil Forming Factors• Soil Components• Soil Properties• Soil Chemistry• Soil Profiles• Pedogenic Regimes2© 2011 Pearson Education, Inc.
  3. 3. Soils• Soil Classification• Distribution of Soils in the United States3© 2011 Pearson Education, Inc.
  4. 4. Soil and Regolith• Thin layer on top of Earth’s crust(about 15 cm deep)• Complex combination of minerals,organisms, gases, liquids, etc.• Nurtures life• Fundamental interface between thefour spheres• Regolith—inorganic material thatresults from rock weathering• Top layer of regolith is different inamount of biochemical weathering4© 2011 Pearson Education, Inc.Figure 12-2
  5. 5. Soil and Regolith• Surface of lithospherealmost always representedby soil• Soil often recognized bycolor• Depth also important, butis only obvious whenexposed throughexcavation5© 2011 Pearson Education, Inc.Figure 12-3
  6. 6. Soil-Forming Factors• Geologic factor– Source of rock fragmentsin soil is parent material– Nature of parent materialinfluences soilcharacteristics– Young soils very reflectiveof the rocks or sedimentsfrom which they resulted6© 2011 Pearson Education, Inc.Image courtesy of
  7. 7. Soil-Forming Factors• Climatic factor– Temperature and moisturesignificant to soil formation– Chemical and biologicalprocesses accelerated byhigh temperatures andmoisture• Topographic factor– Slope and drainagesignificant (Figure 12-5)– Lowering of top and bottomof soil7© 2011 Pearson Education, Inc.Figure 12-5
  8. 8. Soil-Forming Factors• Biological Factor– All soils contain somequantity of livingorganisms– Plant and animalcontributions to soildevelopment– Pedoturbation8© 2011 Pearson Education, Inc.Figure 12-6
  9. 9. Soil-Forming Factors• Biological Factor (cont.)– Earthworms most important• Innumerable tunnels, mixed bywater• Crumbly surface• Digestive actions increaseporosity and resist raindrops,lessening erosion• Casts excreted by earthworms• Rearrange material in the soil• Nitrification promoted– Microorganisms—enhancebiological decay9© 2011 Pearson Education, Inc.Figure 12-7
  10. 10. Soil-Forming Factors• Time Factor– Soil processes very slow– Length of time neededdepends on parent materialand environmentalcharacteristics– Soil degradation or erosioncan occur in only a few years– From human perception oftime, nonrenewable resource10© 2011 Pearson Education, Inc.Figure 12-8
  11. 11. Soil Components• Inorganic materials– Bulk of most soil is mineralmatter in the form of smallbut macroscopic particles– Half the volume of anaverage soil is sand or silt– Smallest particles in soil areclay– Clay has colloidal sizedparticles– Clay attracts cations11© 2011 Pearson Education, Inc.Figure 12-9
  12. 12. Soil Components• Organic matter– Small percent of soil volume– Living organisms, dead organisms, decomposing orcompletely decomposed organisms– Rearrange and aerate soil and yield nutrients throughwaste products– Litter—leaves, twigs, other dead plant parts on soilsurface– Humus—black gelatinous organic matter whichresults from decomposed residues12© 2011 Pearson Education, Inc.
  13. 13. Soil Components• Soil air– Half volume of averagesoil is pore spaces– Spaces provide intersticesamong soil particles– Pores half filled with air,half with water13© 2011 Pearson Education, Inc.Figure 12-11
  14. 14. Soil Components• Soil water– Rainfall, snowfall,groundwater throughcapillary action– Four types– Gravitational water:infiltration from above,falls down via gravity– Capillary water: moisturewhich is held on soilparticle surfaces bysurface tension14© 2011 Pearson Education, Inc.Figure 12-13
  15. 15. Soil Components• Soil water (cont.)– Hygroscopic water: thin film ofmoisture bound to soil particlesby adhesion– Combined water: held inchemical combination with soilminerals– Field capacity and wilting point– Leaching: water carryingnutrients down in soil– Eluviation and illuviation15© 2011 Pearson Education, Inc.Figure 12-14
  16. 16. Soil Components• Soil-water budget– Amount of water added bypercolation and snowmeltdiminished byevapotranspiration– Soil-water balance– In general, cool temperaturesmean less evapotranspirationand water surplus, opposite forwarm temperatures16© 2011 Pearson Education, Inc.Figure 12-15
  17. 17. Soil Properties• Color– Soil color occasionallydescribes nature andcapabilities of soil– Stains on particlesurfaces– Black/dark brown haslarge humus content– Red/yellow indicate highiron oxide content– Gray/blue indicate poordrainage17© 2011 Pearson Education, Inc.Figure 12-16
  18. 18. Soil Properties• Texture– Feel for particle sizes– Separates in sizeclassification groups– Texture triangle(Figure 12-18)• Structure– Particles aggregate intopeds– Soil structure types– Porosity and permeability18© 2011 Pearson Education, Inc.Figure 12-18
  19. 19. Soil Chemistry• Colloids– Particles smaller thanone micrometer indiameter– Inorganic consist of clay– Organic consist of humus– Both are chemicallyactive– Have large water storagecapacity– Attract ions19© 2011 Pearson Education, Inc.Figure 12-17
  20. 20. Soil Chemistry• Cation exchange– Attach to negatively chargedcolloids– Collodial complex– Cation exchange capacity(CEC)– Clay and humus have highCEC activity• Alkalinity– Acid versus base– Alkaline solutions20© 2011 Pearson Education, Inc.Figure 12-20
  21. 21. Soil Profiles• Soil developmentexpressed in twodimensions—time anddepth• Four processes thatdeepen and age soil– Addition– Loss– Translocation– Transformation21© 2011 Pearson Education, Inc.Figure 12-21
  22. 22. Soil Profiles• Horizons—distinct layerswith differentcharacteristics• Vertical profile fromEarth’s surface throughsoil layers—soil profile• Six primary horizons• Solum• Variations on the profile– Fossil horizon– Hardpan22© 2011 Pearson Education, Inc.Figure 12-22
  23. 23. Pedogenic Regimes• Five major soil formingregimes• Laterization– Brick red-colored soil– Requires significantannual moisture surplus– Rapid weathering,dissolution, anddecomposition– Little humus but adequatenutrients– Latosols23© 2011 Pearson Education, Inc.Figure 12-23
  24. 24. Pedogenic Regimes• Podzolization– Gray soil– Limited vegetation nutrient requirements– Acidic plant litter– Effective leaching– Podsols• Gleization– Muddy soil– Poor drainage, acidic– Gley soils24© 2011 Pearson Education, Inc.
  25. 25. Pedogenic Regimes• Calcification– Semiarid climates with little leaching– Little percolating water– Different productivity depending on vegetation• Salinization– In semiarid areas, capillary action brings upwater– Intense evaporation leaves behind salts onsurface– White color to the soil25© 2011 Pearson Education, Inc.
  26. 26. Pedogenic Regimes• Climate and pedogenic regimes26© 2011 Pearson Education, Inc.Figure 12-24
  27. 27. Soil Classification• Soil classification scheme— Soil Taxonomy– Generic classification, hierarchical system– Highest level: soil order– Diagnostic horizons• Epipedon• Subsurface– Second level: Suborders– Third level: great groups• Scale of order and mapping of the soiltaxonomy27© 2011 Pearson Education, Inc.
  28. 28. Soil Classification• The soils of the world28© 2011 Pearson Education, Inc.Figure 12-25
  29. 29. Global Distribution of Major Soils• 12 major types• Entisols– Little profiledevelopment– Due to age orchemical composition• Inceptisols– No diagnostichorizons– Lack of maturity– Common in tundraand floodplains29© 2011 Pearson Education, Inc.Figure 12-27 Figure 12-28
  30. 30. Global Distribution of Major Soils• 12 major types(cont.)• Andisols– Develop fromvolcanic ash– Young with darkupper layers• Gelisols– Young soils thatdevelop slowly due tocold conditions– Found in arctic andsubarctic regions30© 2011 Pearson Education, Inc.Figure 12-29 Figure 12-30
  31. 31. Global Distribution of Major Soils• 12 major types(cont.)• Histosols– Organic, saturatedcontinuously• Aridisols– Soils of dry lands– Distribution correlatedto desert climate31© 2011 Pearson Education, Inc.Figure 12-31Figure 12-32
  32. 32. Global Distribution of Major Soils• 12 major types(cont.)• Vertisols– Little profiledevelopment– Due to age orchemical composition• Mollisols– High in humus andcations– Soft when dry– Found in midlatitudes32© 2011 Pearson Education, Inc.Figure 12-33 Figure 12-34
  33. 33. Global Distribution of Major Soils• 12 major types (cont.)• Alfisols– Subsurface clayhorizon– Medium to generoussupply of cations,nutrients, and water• Ultisols– Weathered andleached of nutrients– Reddish color– Alfisols degenerateinto Utilisols33© 2011 Pearson Education, Inc.Figure 12-35 Figure 12-36
  34. 34. Global Distribution of Major Soils• 12 major types (cont.)• Spodosols– Illuvial dark layerwhere organic matterand other elementsaccumulate– Infertile• Oxisols– Most thoroughlyweathered of soils– Vegetation is efficientin cycling limitednutrients34© 2011 Pearson Education, Inc.Figure 12-37 Figure 12-38
  35. 35. Distribution of Soils in the UnitedStates35© 2011 Pearson Education, Inc.Figure 12-39
  36. 36. Summary• Soil is a thin layer of organic and inorganicmaterial on top of Earth’s crust• Regolith is inorganic material under soil• There are five primary soil forming factors thatgive rise to the global distribution of soils• Soil primarily consists of organic and inorganicmaterial, soil water, and soil air• Each soil has unique color, texture, andstructure36© 2011 Pearson Education, Inc.
  37. 37. Summary• Colloid-cation interaction is important in soilchemistry• Soil acidity is important in its ability to supportplant life• Soil development is expressed in terms ofhorizontal soil horizons• There are five primary pedogenic regimes thatare used to understand the soil-formingprocesses of different soil types37© 2011 Pearson Education, Inc.
  38. 38. Summary• The Soil Taxonomy allows different soil types tobe classified• There are 12 primary soil types, each of whichhas unique color, texture, and structure• Ten of the 12 primary soil types are observed inthe contiguous United States38© 2011 Pearson Education, Inc.