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.

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

A Couple of Soil Myths

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Created 2 September 2011, Last Update 02 September 2011

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