State Factor Model of Soil Formation
State Factor Model
• S = f(C,O,R,PM,T,…)
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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
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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
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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
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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
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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 northcentral U.S., as amount of precipitation increased
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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
20
T a llu la h , L o uisa n na
B la ine C o u n ty, M o ntan a
S o il w a te r h o ld in g ca p a city = 2 2 cm
S o il w a te r h o ld in g ca p a city = 2 2 cm
16
P recipitation
E va p tran sp iration
P recipitation
16
ET
U tiliza tio n
12
D e ficit
8
S u r p lu s
(le a ch in g )
R e ch a r g e
P re c ip ita tio n o r E T , c m
P re c ip ita tio n o r E T , c m
12
D e ficit
8
4
U tiliza tio n
4
0
R e ch a r g e
0
Sep
Nov
Ja n
M ar
M ay
Ju l
Sep
Sep
Nov
Ja n
M ar
M ay
Ju l
Sep
Water Balance Examples
16
Lynn C ou nty, T exas
S ola no C ounty, C alif ornia
S o il a va ila b le w a te r ca p a city = 2 2 cm
A n n u a l P r e cip ita tio n = 6 5 cm
S o il a va ila b le w a te r ca p a city = 2 2 cm
A n n u a l P e cip ita tio n = 6 8 cm
16
P recipitation
ET
12
12
P re c ip ita tio n o r E T , c m
P re c ip ita tio n o r E T , c m
D e ficit
U tiliza tio n
8
8
R e ch a r g e S u r p lu s
4
D e ficit
U tiliza tio n
4
P recipitation
R e ch a r g e
ET
0
0
Sep
Nov
Ja n
M ar
M ay
Ju l
Sep
Sep
Nov
Ja n
M ar
M ay
Ju l
Sep
Biota
• Primary impact on soil development is vegetation (native,
not present)
• Soils developed under grasslands have thick dark surface
horizons
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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
– 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