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 and animals.
In time, mobile minerals and organic particles near the surface will be leached or washed downward and some of them 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.
There are four different processes acting on soil to form 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 processes, distinct layers begin to develop in the parent material and the layers are called soil horizons.
The Pedon
This diagram
above shows that a soil occupies a certain area of the landscape and that
it has depth, width, and length. 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 an airplane 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 have a different name than the surrounding lighter colored
soils.
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.
Information provided by: http://www.soils.agri.umn.edu