Lecture 4b
Soil Forming Factors
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Parent Material
Climate
* Vegetation
* Topography
* Time
Soils vary from place to place because the
intensity of the factors is different at
different locations.
Soil Forming Factors - Biotic (Vegetation)
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Animal - Soil Mixing~ earthworms, crawfish, scorpions, moles,
gophers ~ this mixing can result in the destruction of horizons.
Botic (cont.) – plants - Prairie and Forest
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Vegetation - Additon of Organic Matter (OM).
Prairie ~ OM added to upper 2 ft. of soil due to fibrous root
system of grass plants.
Ap
A
AB
Bg
Biotic (cont.)
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Forest ~ OM added to upper 4 “ due to yearly
leaf fall to surface of soil.
Prairie - Border Biotic Factor
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Prairie - Border soils (oak savannahs) have the influence of
the prairie and forest ~ due to changes in vegetation over
the past 8000 years the soils have been both under prairie
and forest. –
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A horizon is not as dark, not as thick and E may be present or
absent
Ron E. VanNimwegen
Dyad
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Describe your experience of being in a native or restored prairie or an old growth forest. What was unique about this experience?
SFF - Topography or Landscape Position – 3
kinds : 1) Catena, 2) landscape position, 3)aspect
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1) Catena - A
series of soils
with different
horizons due to
differences in
their depth to
the water table
Drainage classes
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Well-drained
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Moderately
well-drained
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Somewhat
poorly
drained
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Poorly
drained
Catena – Natural Soil Drainage Classes
NOTE: Natural drainage refers to depth to water
table not permeability.
 Natural Soil Drainage Classes
 Well Drained - mottles begin > 4 ft.
 Moderately well drained - mottles > 3 ft.
 Somewhat poorly drained mottles > 2 ft
 Poorly drained - dark surface
- gray colors in subsoil below
surface (red mottles)
Drainage Classes (depth to water table)
gray
mottles
gray mottles in brown B
mottles > 4 feet
Well Drained
Mod.well
Dr.
Somewhat Poorly
Drained
gray colors below A
Poorly Drained
2) Topography – erosion & deposition
Summit and Backslope
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Summit will have minimum erosion and maximum soil
development (greatest horizonation).
Backslope will be similar to summit unless slope is > 20%.
Shoulder
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Greatest erosion - least water infiltration greatest runoff - minimal soil development.
Ap
Bw
Bk
BC
C
Footslope
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Deposition of materials from upslope - may be
near water table - may have greatest leaching
due to water from upslope and rainfall.
Ap
A1
A2
A3
AB
Btg
Water
3. Aspect
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Direction the slope faces important when slope is >
than 10 %. Noticeable in
SE MN.
North Slope = 1. colder
soils, 2. less evaporation ,
3. more leaching ~ thus
more soil development
South Slope = 1. warmer
soils, 2. more evaporation,
3. less leaching ~ thus less
soil development.
Aspect
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Direction the slope faces
Important when slope > 10 %
A
E
Bt
C
A
Bw
C
Hillslope in SE Minnesota
North Facing
Slope - more
trees - more
soil development.
South Facing Slope (open, fewer trees)
Less soil development
Soil Forming Factor - Time
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Vegetation and Climate act on the Parent
Material and Topography over Time.
The age of a soil is determined by its
development and not the actual number of years
it has been developing.
How long it takes for a soil to become old
depends on the intensity of the soil forming
processes or intensity of the other 4 soil forming
factors.
Age Sequence
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Youth =
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A
Juvenile =
A
C
Mature =
A
E
Adult
A
E
Old Age
“Senile”
A
E
Bt1
Btqm
Bt
C
Bt2
Bqm
Bw
C
Factors that retard soil profile development
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low rainfall
high lime content
high clay content
steep slopes
cold temperature
severe erosion
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low humidity
high quartz
hard rock
high water table
constant deposition
mixing by animals
Factors that Slow Soil Formation
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Climate
 Low rainfall
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Low humidity
Cold
temperature
Factors that Slow Soil
Formation
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Biota
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Mixing by animals or man
Human made soil Suborder = ARENT
anthropogenic factor – human
influence on soil formation – Direct =
plowing, manuring, liming.
Indirect = changing soil forming
factors – deforestation, relief, parent
material.
The Effects of Management on the Interactions Between Plants and Soils
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In this example, heavy continuous
grazing followed by drought
produces positive feedbacks
between vegetation and soil
properties that enhance physical,
chemical, and biological degradation
processes.
These feedbacks lead to the
following:
Arrow
represent physical, chemical, and biological degradation
a) a decrease in soil organic matter and
an increase in size of bare spaces,
b) a decrease in soil aggregate stability * Desert grass displaced by shrubs. Soil OM decreases and
and resistance to erosion,
Size of bare space increases.
c) a loss of topsoil and a decrease in
* Aggregate stability and resistance to erosion decrease
infiltration,
* Topsoil and fertility lost and infiltration decreases.
d) an additional loss of grass and
increase in shrubs, which cause the
feedback loop to continue.
(Figure modified from Bird et al., 2001.)
What happens to a soil with time
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Loss of nutrients ( bases) = lower pH or soil
becomes more acid
Increase in concentration of iron or soil becomes
redder
Increase in clay content or old soils have more
clay
Deeper weathering into the parent material
Oldest soils in US are on high terraces and
alluvial fans of the mountains in the western U.S.
Oldest soils in United States
Durixeralf - California
Petroargid
Arizona
A
Bk
Bkm
C
Petrocalcic
Petroargid - New Mexico
Callanish Standing Stones – Isle of Lewis - Scotland
In 3000 BC the early farmers plowed the soil where the stones would
be placed. In 1000 BC the stones were forgotten and peat filled in
around the stones to a depth of five feet. When the stones were
discovered in the 18th century, the peat was removed.