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

Salt Crystallization, Utah

Never Safe From Weathering

Weathering Rates

Differential Weathering and Erosion

Surface Area and Weathering

Surface-Volume Effects

Spheroidal Weathering

Spheroidal Weathering and Exfoliation

Spheroidal Weathering

Joints and Spheroidal Weathering

Cavernous Weathering

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

"The 7th Approximation" Degree of Weathering and B Horizon Development LittleSlightModerateLargeExtreme EntisolsAridisols InceptisolsAlfisols SpodosolsUltisols MollisolsOxisols Soils Defined by Special Constituent Materials AndisolsVolcanic Ash HistosolsPeat, Organic Matter Vertisols“Self-Mixing” Clay Soils GelisolsSoils on Permafrost

Soils of the U.S.

Typical Soil Profile (Spodosol)

Aridisol, Kuwait

Ultisols: Alabama Tennessee

Kaolinite, Georgia

Oxisol, California (a Paleosol)

No O Horizon

Paleosol, Scotland

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, Wisconsin

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

Strip Cropping and Contour Plowing