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Weathering and Soils . Weathering Breakdown of Rock near the Surface Due to Surface Processes. Chemical Alteration Solution & Leaching Biological Action Hydration Mechanical Impact Wedging: Frost, Plant Roots, Salt Crystal Growth, Expansion of Hydrated Minerals.
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WeatheringBreakdown of Rock near the Surface Due to Surface Processes Chemical Alteration • Solution & Leaching • Biological Action • Hydration Mechanical • Impact • Wedging: Frost, Plant Roots, Salt Crystal Growth, Expansion of Hydrated Minerals
What Determines Soil Type • Climate • Vegetation • Drainage • Time • Parent Material • Residual - Transported • Least Important Factor for Mature Soils
Soil Formation Processes Leaching from Surface • K, Mg, Na • Ca • Si • Al, Fe Accumulation beneath Surface • Al, Fe in Humid Climates • Ca in Arid Climates
Soil Horizons and Profiles Soil Horizons • Layers in Soil • Not Deposited, but Zones of Chemical Action Soil Profile • Suite of Layers at a Given Locality
Principal Soil Horizons • O - Organic (Humus) Often Absent • A – Leaching • K, Mg, Na, Clay Removed • E - Bleached Zone - Present Only in Certain Soils • B – Accumulation • Absent in Young Soils • Distinct in Old Soils • Al, Fe, Clay (Moist) • Si, Ca (Arid) • C - Parent Material
Limits of Soil Formation 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
Soil Classification This may be the most difficult classification problem in science because of the many factors involved. Varied Bases for Classification • Parent Material • Special Constituent Materials • Maturity • Structure • Climate & Vegetation Multiple Objectives • Scientific • Genesis & Evolution • Agricultural • Fertility • Most Effective Use • Engineering • Slope Stability • Expansion and Shrinkage • Stability of Excavations
"The 7th Approximation" • U.S. Soil Conservation Service • 12 Soil Orders
Loess • Silt-sized • Derived from Glacial Outwash in U.S. • Found in U.S., E. Europe, China • Parent material of world’s prime agricultural soils • Available nutrients • Fine size – lots of surface area • Cohesive – good root support • Porous – retains water well • No Rocks!!!! Windblown silt is called loess
Loess in the U.S. Loess is the raw material for many mollisols, the best agricultural soils
Loess in Europe Loess is the raw material for many mollisols, the best agricultural soils
Loess in China Loess is the raw material for many mollisols, the best agricultural soils
Impacts of Soil Loss • Lessened agricultural productivity • Respiratory hazards (wind erosion) • Siltation of streams • Wetland loss • Destruction of fish habitat • Obstruction of navigation • Eutrophication • Chemical pollution
Controlling Soil Loss • Windrows • Retain vegetation cover • Contour plowing • Strip cropping • No-till agriculture • Sediment dams