Soils We know more about the movement of celestial bodies than about the soil underfoot Leonardo da Vinci 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 Conversion of rock to soil Soil Production Outputs Downslope movement of soil Soil Thickness Storage Soil thickness reflects the balance between rates of soil production and rates of downslope soil movement Slope Weathering Rate Factors of Soil Formation Climate Organisms Parental Material Topography Time Factors of Soil Formation Climate Temperature and precipitation Indirect controls e g types of plants Weathering rates The greater the rainfall amount the more rapid the rate of erosion and leaching 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 erosion agents Plants can physically erode as well as chemically erode Plants stabilize soil profiles Animals including humans tend to increase erosion Factors of Soil Formation Parental Material Chemistry Mineralogy Grain size Factors of Soil Formation Topography Ground slope Elevation Aspect e g north facing vs south facing slopes 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 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 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 Soil Development Additions to Soils Inputs from outside ecosystem Atmospheric inputs Precipitation dust deposition Horizontal inputs Floods tidal exchange erosion landwater movement Inputs from within ecosystem Litterfall and root turnover 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 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 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 Soil Profiles Over time different levels of a soil can differentiate into distinct horizons that create soil profiles Chemical reactions and formation of secondary minerals clays Leaching by infiltrating water Deposition and accumulation of material leached from higher levels in the soil Soil Profiles Cookport soil Pennsylvania A Horizon B Horizon C Horizon Physical weathering breaks rocks into small mineral particles Chemical weathering dissolves and changes minerals at the Earth s surface Decomposing organic material from plants and animals mixes with accumulated soil minerals Parent material solid rock undergoes weathering to become regolith soil saprolite Soil is a mixture of mineral and organic matter lacking any inherited rock structure Soil Saprolite is weathered rock that retains remnant rock structure Saprolite Saprolite 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 Rates of Soil Development U S Department of Agriculture estimates that it takes 500 years to form an inch of topsoil That s less than 0 01 mm yr 1 Erosion of Natural Capital Modern rates of soil loss are 100 to 1000 times rates of soil formation typically mm yr 1 to cm yr 1 in agricultural settings Sets up a fundamental problem due to the erosion of natural capital Soil and the Life Cycle of Civilizations How long would it take to erode 1 m thick soil Thickness of soil divided by the difference between Rate of soil production and erosion 1m 100 to 1000 years 1mm to 1cm 01 mm This is about the life span of most major civilizations Man despite his artistic pretensions his sophistication and his many accomplishments owes his existence to a six inch layer of topsoil and the fact that it rains Author Unknown A nation that destroys its soils destroys itself President Franklin D Roosevelt Feb 26 1937 National Archives 114 SC 5089
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