Lecture 29: Soil Formation
Factors Controlling Soil Formation 1. Parent material: Soil precursor 2. Climate: Temperature and precipitation 3. Biota: Native vegetation, microbes, soil animals, humans 4. Topography: Slope, aspect, landscape position 5. Time: Duration of exposure of parent material to weathering
Parent Material
Soils formed on Aeolian Deposits
Grassed Sand Hills of Nebraska
Loess Windblown silt deposits, typically found in areas near glaciers or draining glacial meltwater Becomes productive farmland after soil development Highly susceptible to erosion From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Thick Loess Deposits in Central U.S.
Organic Deposits Accumulation of organic matter in wetlands leads to organic soil formation Forms in bogs, fens, swamps, and marshes Often called Peat
Key Concepts in Parent Material Parent material shows geographic variations across a country Multiple origins for parent material Residual (formed in place from rock weathering) versus transported Common transported parent materials include colluviums, alluvium, glacial material, and aeolian deposits (sand, dust, and loess)
Climate
Biomes of N. America
Soil Orders Vary with Climate
Global Atmospheric Circulation Dry Wet Dry Wet Dry Wet Dry
Variation in Weathering Depth with Latitude From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Intertropical Convergence Zone (ITCZ)
Intertropical Convergence Zone (ITCZ): Zone of Oxisols
Aridisol-Mollisol-Alfisol Climosequence
Key Concepts in Climate Climate affects soil formation by controlling the amount of rainfall and the temperature This is reflected in the dominant vegetative cover Different soil types dominate different climate zones, but there are always exceptions The deepest weathering occurs in equatorial and temperate regions because that is where rainfall is concentrated Regions often display clear climosequences
Biota
Islands of Fertility in Arid Environments
Pedoturbation by Animals
Influences of Biota: Organic Matter Accumulation OM deposited by roots in A horizon OM deposited as leaves on the soil surface
Influences of Biota: Cation Cycling Conifer needles are low in Ca 2+ Little cation recycling occurs Organic matter on soil surface becomes acidic with low base saturation Lower uptake of Ca 2+ from deep in the soil may lead to slower weathering Deciduous leaves contain substantial Ca 2+ This is recycled into the surface soil, reducing acid saturation Deciduous trees effectively leach Ca 2+ from minerals, increasing weathering lower in the soil profile
Difference in Base Cation Cycling of Vegetation Affects Soil Formation Spodosol Alfisol Mollisol From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Key Concepts in Biota Biota affect soil formation by: Localizing OM and soil development Mixing the soil Inputting OM and organic acids and removing water via transpiration Trees cycle cations through the soil Cation availability affected by ability of plants to cycle base cations
Topography
Upland Welldrained, thick soil Slope Well-drained, but thin from erosion Lowland Thick soil, but poorlydrained, shallow water table Topography
Interaction of Topography and Parent Material Residual, colluvial, and alluvial parent materials occupy distinct landscape positions
Interaction of Topography and Parent Material Typical Soil Association in the Appalachian Mtns., USA
Toposequence in Volcanic Terranes From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Time
Approximate Time to Form Soil Features General formation curves compiled from studies of chronosequences The time required to form diagnostic horizons and their associated soil orders varies from 10 to 1,000,000 years From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Soil Development on Granite Material in a Warm, Humid Climate Representative of Northern Georgia
Soil Development on Calcareous Loess in a Warm, Subhumid Climate Representative of Central Nebraska
Key Concepts in Topography and Time Topography acts as a large-scale control that modifies the other soil forming factors Affects rainfall and temperature, which affects the biota present and the extent of weathering Soil development is fundamentally a temporal process The more time the soil forming factors have to act, the greater the soil development Clear time-series progressions of soil development are seen in specific climates
Weathering: Making Soil from Parent Material
Weathering: Turning Parent Material into Soil
Physical and Biogeochemical Weathering A combination of physical and chemical weathering causes exfoliation Chemical weathering (mostly oxidation) produces concentric bands in this cobble
Relative Importance of Chemical and Physical Weathering depends on Climate Mollisols Alfisols Aridisols Ultisols Oxisols
Biogeochemical Weathering Reactions Congruent Dissolution: Mg 2 SiO 4,forsterite + 4H + = 2Mg 2+ + H 4 SiO o 4 Incongruent Dissolution: 2NaAlSi 3 O 8,albite + 9H 2 O + 2H + = Al 2 Si 2 O 5 (OH) 4,kaolinite + 2Na + + 4H 4 SiO o 4 Oxidation-Reduction: 4Fe 2+ + 6H 2 O + O 2 4FeOOH goethite + 8H + Complexation/Chelation: 2H 2 C 2 O 4 + NaAlSi 3 O 8,albite + 4H 2 O = Al(C 2 O 4 ) + + Na + + C 2 O 2-4 + 3H 4 SiO o 4
Weathering Changes Composition Granite Gneiss and the Soil Formed Above It
Weathering Alters Mineralogy
Soil Orders Vary with the Degree of Weathering
Key Concepts in Weathering Weathering occurs through both physical and biogeochemical processes These physically breakdown parent material into smaller pieces and transforms it into new minerals Weathering change the composition and mineralogy of a system Clay mineralogy reflect the extent of weathering and the climate Soil orders vary in the extent of weathering that has occurred