Soil generally consists of visually and texturally distinct layers, which can be summarized as follows from top to bottom:
O) Organic matter: Litter layer of plant residues in relatively undecomposed form.
A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminum, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored "E" subsurface soil horizon is apparent at the base of the "A" horizon. A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A-horizon is regarded as a "biomantle".
B) Subsoil: Layer of alteration below an "E" or "A" horizon. This layer accumulates iron, clay, aluminum and organic compounds, a process referred to as illuviation.
C) Substratum: Layer of unconsolidated soil parent material. This layer may accumulate the more soluble compounds that bypass the "B" horizon.
ACKNOWLEDGEMENT: “Borrowed” from Wikipedia