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Soils

Soils. Soil: Definition. Solid earth material that has been altered by physical, chemical and organic processes so that it can support rooted plant life. Engineering definition: Anything that can be removed without blasting. Soil Production. Soil Production: Inputs.

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Soils

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  1. Soils

  2. Soil: Definition Solid earth material that has been altered by physical, chemical and organic processes so that it can support rooted plant life. Engineering definition: Anything that can be removed without blasting

  3. Soil Production

  4. Soil Production: Inputs Conversion of rock to soil

  5. Soil Production: Outputs Downslope movement of soil

  6. Soil Thickness: Storage input ± output = soil thickness or: rock weathering ± soil transport = thickness Soil thickness reflects the balance between rates of soil production and rates of downslope soil movement.

  7. Factors of Soil Formation • Climate • Organisms • Parental Material • Topography • Time

  8. Factors of Soil Formation • Climate • Temperature and precipitation • Indirect controls (e.g., types of plants) • Weathering rates • The greater the rainfall, the faster the rates of erosion and leaching.

  9. Factors of Soil Formation • Organisms • Types of native vegetation • Weathering is dependent of plant growth • Plant and animal activity produces humic acids that are powerful weathering agents. • Plants can physically erode as well as chemically erode. • Plants stabilize soil profiles, Animals (including humans) tend to increase erosion.

  10. Factors of Soil Formation • Parental Material: • Chemistry • Mineralogy • Grain size

  11. Soil Texture

  12. Soil Texture

  13. Factors of Soil Formation • Topography: • Ground slope • Elevation • Aspect (e.g., north facing vs. south facing slopes)

  14. Factors of Soil Formation • Downslope transport of soil is a function of slope: • Erosion rate = f(S) The steeper the surface slope, the more likely any eroded material is to be transported out of the system.

  15. Factors of Soil Formation Soils on hillslopes reach an equilibrium thickness, often about 1 m. Soils on flat surfaces, such as floodplains or plateaus, tend to thicken through time due to weathering rates being greater than sediment transport rates.

  16. Factors of Soil Formation • Time • Development and destruction of soil horizons • Reaction rates are slow, the longer a rock unit has been exposed, the more likely it is to be weathered.

  17. Soil Development

  18. Additions to Soils Inputs from outside ecosystem • Atmospheric inputs • Precipitation, dust, deposition • Horizontal inputs • Floods, tidal exchange, erosion, land-water movement Inputs from within ecosystem • Litterfall and root turnover

  19. Transformations • Decomposition of organic matter • Humification to form complex organic matter • Weathering of rocks to produce more stable forms • Physical weathering • Fragmentation of rock • Freeze-thaw; drying-wetting; fire • Physical abrasion • Abrasion by glaciers • Chemical weathering • Dissolves primary minerals • Forms secondary minerals

  20. Decomposition • Breakdown of soil organic matter to form soluble compounds that can be absorbed or leached • Depends on • Quantity of input • Location of input (roots vs. leaves) • Environment • Temperature • moisture

  21. Soil Horizons and Profiles Soil Horizons • Layers in Soil • Not Deposited, but zones of chemical action Soil Profile • Suite of horizons at a given locality

  22. Soil Horizons • Over time, soil layers differentiate into distinct ‘horizons’ defined by zones of chemical action. Chemical reactions and formation of secondary minerals (clays). Leaching by infiltrating water (elluviation). Deposition and accumulation of material leached from higher levels in the soil (illuviation). Soil Horizons and Profiles

  23. Saprolite is weathered rock that retains remnant rock structure. Soil Saprolite Bedrock

  24. Weathering Products:Product of weathering is regolith or soilRegolith or soil that is transported is called sediment

  25. Weathering often proceeds along fractures and joints, resulting in progressive development of corestones.

  26. Weathering often proceeds along fractures and joints, resulting in progressive development of corestones that when exposed at the surface can form tors and larger bornhardts (also known as inselbergs or rock islands).

  27. Inselberg (bornhardt), Mali

  28. Inselberg (bornhardt), Ayers Rock, Australia

  29. Inselberg (bornhardt), precambrian granite, central Texas

  30. Typical soil profile

  31. Cookport soil, Pennsylvania A Horizon B Horizon C Horizon

  32. Soil classification = Soil orders Aridisols = arid zone soils (calcic horizons) Mollisols = grassland soils (thick A horizon) Alfisols, Ultisols, and Spodosols = forest soils (thick B horizon) Oxisols = tropical soils (quite oxidized) Histosols = wetland soils Gelisols = polar soils Andosols = volcanic parent material Vertisols = swelling clays Entisols = weak A over C horizon Inceptisols = weak B horizon

  33. Soil classification = Soil orders Aridisols = arid zone soils (calcic horizons) Mollisols = grassland soils (thick A horizon) Alfisols, Ultisols, and Spodosols = forest soils (thick B horizon) Oxisols = tropical soils (quite oxidized) Histosols = wetland soils Gelisols = polar soils Andosols = volcanic parent material Vertisols = swelling clays Entisols = weak A over C horizon Inceptisols = weak B horizon

  34. Soil map of world Greens – Alfisols & mollisols Red – Oxisols Tan - Aridosols

  35. Soil types: mollisols Decomposing organic material from plants and animals mixes with accumulated soil minerals.

  36. Alfisols and mollisols: Form in warm or cool, temperate climates. Soil is clay-rich and fertile. Alfisols are forest soils Mollisols are grassland soils

  37. Soil types: aridisols Physical weathering breaks rocks into small mineral particles.

  38. Aridisols Route 66

  39. Soil types: oxisols Intensive leaching and oxidation combine to create iron-rich residual soils.

  40. Soil types: oxisols Laterite = highly developed oxisol. Forms in a hot, humid climate. Soil is deep red, hard and infertile.Plants recycle nutrients in a thin A and O horizon. Laterite formation on gneiss, Kerala, India

  41. Laterites

  42. Deforestation removes the fertile organic layer. The underlying soil is infertile, dries to brick-like hardness when it dries out, and is difficult to cultivate. Aluminum (from bauxite) and iron (from limonite) can be mined from these soils.

  43. Carajas, Brazil

  44. Limits of Soil Development Balance Between: • Downward Lowering of Surface • Downward Migration of Horizons If erosion rapid or soil evolution slow, soils may never mature beyond a certain point. Extremely ancient soils may have lost everything movable

  45. Weathering with Climate Temperate Tropical Arid Polar

  46. Soil Production Function

  47. Sugar Loaf, Rio De Janiero, Brazil

  48. Convex, soil-mantled hillslopes, Northern California

  49. Agricultural Soil Loss Erosion of unprotected soils during periods where they remain fallow and without vegetative cover leads to increased erosion

  50. We know more about the movement of celestial bodies than about the soil underfoot. - Leonardo da Vinci

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