Weathering and Soils

WeatheringBreakdown of Rock near the Surface Due to Surface

Processes Chemical Alteration • Solution & Leaching • Biological Action • HydrationMechanical • 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

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 ProcessesLeaching 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 ActionSoil 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 FormationBalance 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 ClassificationThis 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 DevelopmentLittle Slight Moderate Large ExtremeEntisols Aridisols

Inceptisols AlfisolsSpodosols UltisolsMollisols Oxisols

Soils Defined by Special Constituent MaterialsAndisols Volcanic AshHistosols Peat, Organic MatterVertisols “Self-Mixing” Clay SoilsGelisols Soils on Permafrost

Soils of the U.S.

Typical Soil Profile

(Spodosol)

Aridisol, Kuwait

Ultisols:AlabamaTennesse

e

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

Strip Cropping and Contour Plowing