State Factor Model of Soil Formation

1. State Factor Model of Soil Formation

2. State Factor Model • S = f(C,O,R,PM,T,…) • 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

3. “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

4. 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

5. 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.

6. 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  

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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.

13. 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.

14. 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…

15. 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

16. Water Balance Examples

17. Water Balance Examples

18. 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

20. 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.

22. Time • Passive factor • 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

23. 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

24. Relief: slope gradient, landscape position