1. State Factor Model of Soil Formation

2. 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

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 ( 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 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

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