State Factor Model of Soil Formation

State Factor Model S = f(C,O,R,PM,T,…) Model has many shortcomings C – climate O – organisms R – relief PM – parent material T – time  Model has many shortcomings It has never been solved mathematically and probably never will be. It oversimplifies the complexity of soil formation. It implies that four of the factors can be fixed and one varied to observe the effects of the one variable on rates or kinds of soil formation processes.  Many studies have attempted to fix four of the factors to evaluate the influence of the fifth

“Sequence” Studies Toposequence: vary landscape position Climosequence: vary climate Chronosequence: vary age Stream terraces Biosequence: vary vegetation Lithosequence: vary parent material These approaches ignore interactions among the factors The state factor model helps understand differences among soils Parent material and relief are passive factors Climate and organisms are active (or flux) factors Add materials to the soil Drive processes Time allows the other factors to act

Parent Material Rock or sediment from which the soil develops has a strong influence on the properties of the soil that forms The mineral components in the soil and its chemical and physical properties depend on: Mineral components in the parent material If precursor for a secondary mineral is not present in the parent material, the soil will never contain the secondary mineral Plagioclase feldspar  smectite K feldspar  kaolinite Time and environmental conditions of the weathering environment Plagioclase feldspars  smectite  kaolinite

Parent Material – Soil Relationships Light colored crystalline rocks (granite and granitic metamorphic rocks: felsic) Common parent material in the Piedmont and Blue Ridge Mountains Dominant minerals K feldspar, quartz, mica (biotite or muscovite) Rock weathers to saprolite with low clay content Soils derived from saprolite are Sandy A / clayey B kaolinitic, Moderately permeable, acidic, and have low base saturation and nutrient reserves.

Parent Material – Soil Relationships Dark colored crystalline rocks (gabbro, basalt, and metamorphic counterparts: mafic) Dominant minerals Amphibole, pyroxene, and plagioclase feldspar K feldspars, mica, and quartz are minor components Soils developed from these rocks or saprolite Loamy/silty A / v. clayey B Appreciable Fe-oxide minerals and are often dark red Smectite clays; Less acidic; Higher base saturation and higher productivity  

Sedimentary Deposits Loess - windblown silts Common along rivers carrying meltwaters from glaciers Properties of loess and resulting soils depends on rocks passed over by the glacier Properties also vary with with distance from source area Silty soils, or silty caps over underlying materials Thinner deposits with more clay as distance from the source increases

Sedimentary Deposits Glacial till - material deposited by glaciers and processes related to glaciation Most common parent material in the midwest of North America and over much of Europe Properties reflect the properties of rock passed over by the glacier Midwest - limestone and shale Loamy soils with high pH, high base saturation, and smectitic clays Northeast - granite and acid sandstone Acid soils with sandy loam texture and low base saturation

Coastal Plain Sediments Associated with marine and near shore environments Properties depend on sediment source environment of deposition beach and dunes - eolian sands; little silt and clay riverine deposits – texture varies depending on position in the floodplain deltaic deposits - variable texture depending on depositional environment shallow marine - carbonate minerals mixed with terrestrial materials amount of terrestrial material depends on distance from shore and shelf position Except for limestone, sediments were derived from upland erosion of previously formed soils Previous weathering

Coastal Plain Sediments Limestone – rock with >50% carbonate minerals (calcite and dolomite) Carbonates dissolve during soil formation In humid climates, all of the carbonates dissolve and are leached In semi-arid and arid climates, incomplete leaching results in the carbonates being re-distributed in the soil and concentrated in subsoil horizons Silicate minerals composing the soil were impurities in the limestone May be as little as 2-5% of the rock Shallow marine deposits - silicates are clay-sized Limestone derived soils are often clayey

Coastal Plain Sediments Sandstone – rock composed of sand-sized minerals Quartz is often the dominant mineral Varying amounts of more weatherable minerals Depends on mineralogy of sand source Properties of soils derived from sandstone depend on the composition of the sandstone Shale – rock composed of clay-sized grains Composed of clay minerals, quartz, and feldspars Shale derived soils are clayey.

Relief Primary effect is its influence on hydrology Water moves downhill, often laterally In humid climates, lower landscape positions generally have seasonal water tables Convex positions have more runoff and erosion than planar or concave positions More runoff = less water infiltration = less soil development Enhanced erosion also removes surficial soil and retards development  Concave positions accumulate water and sediment Over-thickened A and E horizons Thick A horizons due to slower organic matter decomposition Shallow subsurface water movement may carry mobile constituents to lower topographic positions.

Climate Solar radiation Temperature Precipitation Water is the driving force for soil formation Climate effects are primarily related to the intensity of leaching and the amount of biomass production. Across a precipitation gradient (380-980 mm) in the north-central U.S., as amount of precipitation increased pH decreased depth to carbonates increased N content of soil increased clay content increased.

Climate Rainfall amount greatly affects weathering, leaching Low rainfall (< 20”/yr): low rate of weathering, limited clay formation Moderate rainfall (20-30”/yr): 2:1 clays stable (rate of base cation leaching low) Higher rainfall (40-50”/yr): 1:1 (kaolinite) favored due to loss (over time) of basic cations 2:1’s may persist if low Ksat (due to swelling clays) limits leaching … Very high rainfall (70-100”/yr): intense weathering, leaching Kaolinite weather to gibbsite (clay destruction)—loss of Si Fe oxides accumulate This is all affected by time over which rainfall occurs…

Climate Polar climates - freeze-thaw cycles produce ice wedges and frost heaving in polar climates Cold temperatures can also slow weathering reactions Soils on very old landscapes in Antarctica do not have Bt horizons because weathering reactions are slow Temperature influences the type and quantity of vegetation in an area Amount and quality of organic matter Water balance controls amount of water available to drive soil formation and the depth to which leaching occurs Net precipitation or rainfall surplus = precipitation - evapotranspiration “Average" or extreme events

Water Balance Examples

Water Balance Examples

Biota Primary impact on soil development is vegetation (native, not present) Soils developed under grasslands have thick dark surface horizons fibrous root system a greater proportion of the biomass of grasses is in roots As roots die, the organic matter is in the soil. higher lignin content and are more resistant to decomposition Soils developed under hardwoods have thinner A horizons tap root system Organic matter concentrated in a limited area Leaves fall to the surface Other mechanisms are needed to incorporate decomposing leaves into the soil

Biota Conifers/high-tannin plants: soluble humic compounds lead to formation of Bh horizons Form in sandy deposits with high seasonal water table Redox and chelation combine to form bleached albic E horizons Humics and Fe precipitate to form Bh, Bhs horizons Occurs at surface of high water table (re-oxidation) In GA: live oak/palmetto on v. sandy marine deposits (Flatwoods) Humans have also had an appreciable impact on soil development through agriculture, mining, and other soil disturbing activities.

Time Passive factor Over time, the possible fates for the soil are: Only impact is to allow the two active factors, climate and biota, to express themselves Over time, the possible fates for the soil are: continue indefinitely in its current form rate of erosion = rate of soil formation become more developed rate of erosion < rate of soil formation become the parent material for another soil existing soil modified by a new set of processes in a new environment become buried by a new parent material disappear - be eroded to become parent material for a new soil

Time What is the rate of soil formation? “it depends” “it is a combination of factors” Rate depends on the interaction of the other four state factors “Rapid Processes” (a few decades to a few hundred years): A horizon formation structure formation leaching of water soluble components in humid climates “Intermediate Processes” (a few thousand years): subsoil organic matter accumulation (Bh horizon formation) subsoil carbonate accumulation (Bk horizon formation) “Slow Processes” (a few 10’s of thousand years) clay translocation (better considered to be many thousands, i.e. 7-10) induration of subsoil by carbonates, Fe oxides, and other mobile components

Relief: slope gradient, landscape position