Soil profiles refer to the top layers of earth and their constituent parts. A change in structural material, one upon the other, delineates each layer or horizon, until you reach the bedrock. The profiles will differ from place to place, depending upon where in the environment the sample comes from. The names A, B, and C are given to the profiles with respect to their sequence from the surface. The O and E horizons gain their labels from their specific contents.
The O horizon is composed of decomposing organic material that has settled upon the topsoil. As this material breaks down it becomes humus. It is distinct from the A profile in that it has not settled into the topsoil and contains no mineral components.
The topsoil represents the A horizon, created by the weathering of larger material. In the absence of organic materials, this is the first visible layer of soil profiles. Its depth will vary with the environment and its constitution can change over time by eluviation. . This is the process of material, both organic and inorganic, moving through the soil, suspended or dissolved in water. Eluviation will not only mix organic with mineral material as it drains through the O horizon, it will also leach both of these from the A horizon and carry it to lower profiles.
An E horizon is one significantly depleted of organic and mineral components by eluviation. The E stands for eluviated. This layer is mainly composed of silicates characteristic of the geological sources of the area as other materials leached away.
The product of eluviation is an illuvial layer of soil known as the B profile. The accumulation of mineral solids leached from the layers above form this stratum. It is generally a dense soil, distinct in color and texture from that above it. Its composition will vary with environmental factors, especially the amount of water affecting the upper layers of the soil.
The C horizon is defined differently, depending on the taxonomy used to define the soil profiles. It is either the bedrock beneath the soil, or the first layer above the bedrock whose main component is parent material. Weathering has had little effect on it, but geological activity may have transported it to its location.