Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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Parent material is
broken bown into smaller pieces through physical and chemical
Physical processes include freezing and thawing of water, plant root growth,
physical transport of material
Chemical processes are
caused by decomposition of rock by rainwater and organic acids. Over time, rock is
turned into clay, and chemicals are leached away. The higher the acidity of
inflowing water, the more rapidly this will happen. The major sources of water
acidity are from rain itself, and from organic acids found in decaying organic
Soils will be richer
and thicker at the bottom of a hill
Soils will become more
leached and thick as precipitation increases
Soils will become more
leached and thick as time increases
Geology and soil fertility
The ability of nutrients to
'stick' to soil varies between soils developed into different parent material.
Parent material is not only bedrock, but any abiotic material which can
decomposed into soil (like glacial drift or sand dunes).
Soils developed from acidic
igneous rock (rhyolite, granite), sandstone, or shale, have the lowest
ability to retain nutrients
Soils developed into basic
igneous rock (basalt, gabbro) have higher ability to hold onto
Soils which have lots of
organic matter have the highest ability to retain soil nutrients.
However, some organic rich soils (peat bogs) have low nutrient levels, as none
have been leached into system.
Soils are classified as residual (developed from underlying bedrock) or
transported (developed from loose material transported by water, glaciers or
wind. It's obviously a lot easier to develop soil if the parent material has
already been broken up. Virtually all soil in Wisconsin is transported.
Soil usually consists of a number of discrete layers, or horizons:
O-Horizon is at
surface, and is made up of undecomposed and decomposed organic
A-Horizon lies below O,
and is made of highly decomposed mineral material mixed with highly decomposed
organic material (humus). Often, some leaching occurs in this
If leaching is intensive, a very light layer can form which is called the
E-horizon. Not all soils have an E-horizon.
B-Horizon has no
organic material, and is made of highly decomposed parent material. In many
cases, the materials leached from the A-Horizon (primarily clays and iron oxide)
are deposited here
C-Horizon is made up of
partially-decomposed parent material
With increasing depth, soil
generally becomes less leached and less developed.
parent material is called the R-Horizon (for "rock").
Soils and Climate
Soils form though
interactions between an area's geology, climate, and biota.
Geology will determine the
type of parent material, what it will decompose into, and the ability of soil to
Climate (ratio of
precipitation to evaporation) will determine the degree and intensity of
Vegetation will also determine
the leaching rate (if dead leaves are acidic, leaching rates will be high), the
rate of physical breakdown of parent material, and the ability of soils to
Thus, each major climate and
life zone on Earth will have different soils.
The US Department of Agriculture classifies soils into twelve orders.
- Seasonal climates with
more rainfall than evaporation and with deciduous tree cover will form the
'typical' soil profile (called an alfisol)
- Cold or seasonal climate
with acidic parent material, much more precipitation than evaporation, and
coniferous tree cover will form a spodosol. As conifer leaf litter is very
acidic, leaching rates will be very high, leading to a very strongly bleached
lower A-Horizon and a dark upper B-Horizon where the leached material is
deposited. These soils are characteristic from Green Bay north to Hudson's Bay
where Precambrian granites are exposed.
- Climates which are warm
year-round, and in which precipitation is very much greater than evaporation
form oxisols in the tropics, or ultisols ("ultimate" soils) in
temperate climates like the southern U.S. Everything that can be removed
has been, so these soils are very poor in nutrients. In oxisols, leaching is so severe that only iron and
aluminum oxides are not leached away, forming a brick-like substance called
laterite. As essentially no nutrients are found in these soils, nutrients must
be stored in the vegetation. This is why tropical forests will not recover after
they are cut down and the vegetation taken away. Tropical forests are lush
because they ruthlessly recycle nutrients. There is no humus in tropical
soils; the leaf litter lies directly on bare soil. This deceptive
appearance has led to rain forests being called a "counterfeit
- Climates which have
slightly more evaporation than precipitation, and grassland vegetation, will
form a mollisol. Because there can be more evaporation than precipitation, no leaching
occurs. Nutrients are drawn up from parent material and retained in the A-Horizon,
which becomes very thick. These soils created the rich farmlands of the central
U.S., the Ukraine, China, Argentina. It is no exaggeration to say mollisols feed
- Desert soils evolve slowly and only calcium leaches out to any great
extent. It doesn't get very far, but cements the subsoil into a hard
layer of calcium carbonate called caliche. Desert soils are
- The other types:
- Peat soils are called histosols
- Soils formed from volcanic materials are called andisols
- Soils formed on permafrost are called gelisols
- Some soils are made of clay that expands and contracts with
wetting and drying. These are called vertisols.
- Materials that are so young they have no appreciable soil
formation are called entisols.
- Materials that are just beginning to show evidence of soil
formation are called inceptisols.
Entisols are typically less than a few hundred years old, inceptisols a
thousand or so, alfisols and mollisols perhaps 10,000, ultisols and oxisols
can be 100,000. Soils rarely get older than that because erosion eventually
strips off soil as fast as it forms.
In areas long farmed, soil profiles have been extensively modified by
humans. Such soils are termed anthropogenic.
Importance to Humans
- Mollisols and alfisols are the world's chief agricultural soils
- Young andisols (volcanic soils) can be very nutrient rich and
are important agricultural soils in places
- Ultisols and oxisols are deceptive. The long growing season
in warm climates misleads people into thinking they are very productive, but they can
be quickly depleted of nutrients if not managed well.
- Aridisols can be productive if irrigated, but if erosion isn't
controlled, the hard caliche layer can be exposed. As one soil
scientist put it "If that happens, your next crop rotation better be
- Vertisols can be productive. Their principal importance is
engineering. The expansion and shrinking of the soil can damage
roads and foundations.
- Histosols can be productive until their stored nutrients are
used up. Entisols and inceptisols can be productive if they leach
nutrients fast enough to be utilized by crops.
A Couple of Soil Myths
- You can't identify a soil merely from surface material. The soil
type is determined by the whole profile. So detective shows where
someone identifies a soil type from a bit of dirt on a shoe are pure
- Soil mapping is expensive and time consuming. Soil maps are not
detailed enough to allow the location of a soil sample to be
determined with high accuracy, and soils are determined by
vegetation, drainage and climate, so a given soil type occurs over a
fairly wide area. More bad news for CSI. (I once saw a CSI episode
where quartz sand turned up on a shoe and the investigator said
"He's been in the desert." In Las Vegas. Ya think?
- People often ask me if geologists can tell what the bedrock
is by mapping the soil. Oh, I wish. First, soil won't reveal
bedrock beneath 100 feet of glacial deposits. Second, in areas
of old soils, climate, vegetation and drainage dominate so
identical soils can result from very different bedrock. However,
in areas of deep and very ancient residual soil, it is sometimes
possible to discern original features from the bedrock preserved
in the soil, especially in road cuts and excavations.
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Created 2 September 2011, Last Update
02 September 2011
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