When rocks on the surface of the earth weather, eventually enough essential elements become available to support lichens and other lower forms of plant life. As continuing generations of lichens grow, die, and decay, they leave increasing amounts of organic matter. Naturally-occurring organic acids further hasten decay of the rock. An increasing build-up of organic matter and formation of fine rock fragments result in more water retention in the soil and more water available for use by larger numbers of plants.
In time, mobile minerals and organic particles near the surface will be leached or washed downward and some of them will be deposited a few inches below the surface. This zone of depositions constitutes the beginning of an accumulation soil layer. After a few hundred years, the leached surface soil layers will be visible and are called zones of eluviation. The subsoil may eventually be well developed (a zone of illuviation) and will be considerably different from the surface soil layer because of the soil forming processes.
There are four different processes acting on soil to form soil horizons. These general processes are additions, losses, translocations and transformations.
Processes
Additions include the adding of: organic
matter from plants, water, air, and energy from the sun.
Losses include: water due to evaporation or
transpiration and nutrients leaching from the soil or being taken up by plants.
Translocations include the movement of:
clay, organic matter, salts, and nutrients from one layer to another.
Transformations include: the formation of
clay, and the arrangement of the soil components into structural aggregates.
Because of these soil forming processes, distinct layers begin to develop in the parent material and the layers are called soil horizons.
This diagram shows that a soil occupies a certain area of the
landscape and that it has depth, width, and breadth. This soil we see in a soil
pit is called a "soil pedon". If we stand in the pit and look at each
pit wall we will see that the layers are the same on each wall.
When we observe a cross-section of the soil from the surface down to the underlying material, we say that we are observing the soil profile. The layers in the soil profile are referred to as horizons. These horizons are the zones of eluviation (leaching out) and illuviation (washing in). Horizons are formed by various processes that act on the soil.
Soils
have east-west and north-south extent besides depth. Notice the dark strip in
the middle of this field. It is lower than the surrounding soils and therefore
has developed different characteristics. The characteristic most noticeable is
a darker surface horizon. The soil scientist making a soil map of the field
would draw a soil boundary around this low area.
If we look at this area from above by using an aerial
photograph, we can see how easy it is to identify the exact location of this
darker surface soil. The soil will have different characteristics such as being
poorly drained and thus will be given a different soil name (dark soil=Webster)
than the surrounding lighter colored soils (lighter soil=Clarion).
The concept of a soil having depth, width, and breadth is called the soil body. Each body is made up of a series of soil profiles called a pedon - and collectively, pedons are called polypedons. Polypedons make up the soil mapping unit that soil scientists draw on the soil survey map. Soil surveys will be studied in a later unitSoil Survey Laboratory.
How long does it take for soils to develop?
Information from Rick
Cruse at the University of Iowa found that C-N-W soils are about 36”deep
and the C-N-W soils are about 14,000 years old. So each year how many tons of
soil developed?
Answer=36 in/14,000 years=0.003 in/year. This is equal to about 1,000 lbs. per
acre per year. What their study determined was that the rate of erosion for the
C-N-W soils is faster then the rate of erosion. (http://extension.agron.iastate.edu/soybean/documents/SoilErosion.pdf)
Soil Forming Factors -Chapter 2 - Climate and Biotic
© Regents of the University of Minnesota, 2007 The University of Minnesota is an equal opportunity educator and employer.