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Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Soil Genesis

Parent material is broken bown into smaller pieces through physical and chemical processes.

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 matter.

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 nutrients

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 Profile

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 matter

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 zone. 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.

Sometimes the 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 retain nutrients.

Climate (ratio of precipitation to evaporation) will determine the degree and intensity of leaching

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 retain nutrients.

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 paradise."
• 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 the world.
• 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 called aridisols.
• 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 oil."
• 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 fiction.
• 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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