The arrangement of horizons in a soil is known as a soil profile. A soil horizon is a layer generally parallel to the soil crust, whose physical characteristics differ from the layers above and beneath. Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material (i.e., "coarser" or "sandier" than the horizons above and below). The differentiation of the soil into distinct horizons is largely the result of influences, such as air, water, solar radiation and plant material, originating at the soil-atmosphere interface. Since the weathering of the soil occurs first at the surface and works its way down, the uppermost layers have been changed the most, while the deepest layers are most similar to the original parent material.
HOW THE SOIL PROFILE DEVELOPS
The soil profile is developed by the five (5) soil forming factors which are; Climate, Organisms, Relief/Topography, Parent material and Time
The Horizons
Soil generally consists of visually and texturally distinct layers, also called profiles, which can be summarized as follows;
O) Organic matter
Surficial organic deposit with litter layer of plant residues in relatively non-decomposed form.
A) Surface soil
Organics mixed with mineral matter. This layer of mineral soil contains the 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".
E) Horizon
"E", being short for eluviated, is most commonly used to label a horizon that has been significantly leached of clay, iron, and aluminum oxides, which leaves a concentration of resistant minerals, such as quartz, in the sand and silt sizes. These are present only in older, well-developed soils, and generally occur between the A and B horizons.
B) Subsoil
Subsurface layer reflecting chemical or physical alteration of parent material. This layer accumulates iron, clay, aluminum and organic compounds, a process referred to as illuviation.
C) Parent rock
This layer is also known as substratum. The parent material in sedimentary deposits. Layer of large unbroken rocks. This layer may accumulate the more soluble compounds.
R) Bedrock
The parent material in bedrock landscapes. This layer denotes the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer. These areas of bedrock are under 50 feet of the other profiles.
