The U.S. Soil Taxonomy
& Mississippi Soils

Information modified from Mississippi
Extension Service & other materials

Soils Up and Down - The Soil Profile
The overall vertical view of a soil is called the profile. Long-term processes of soil development and weathering, result in the formation of distinctive layers called horizons that are visible in the profile. Horizons reflect the cumulative effect of all the soil formation factors. The top layers, often referred to as topsoil, are often two different horizons. If there is a layer of partially decomposed plant material, it is designated the O or organic horizon. Below the humus, there is usually a dark layer several inches thick called the A horizon. Soil scientists note that if a soil has ever been manipulated by humans (plowed, bulldozed, shoveled, raked, etc.), a subscript is added, i.e. Ap. This plow layer is the primary rooting zone and nutrient supply area for plants. In well-developed soils, an area just below the A horizon can be seen as a light-colored, often diffuse band - the E horizon. The "E" is the zone of maximum eluviation and represents the transition to the B and lower horizons.

Lower horizons show different degrees of development (soils weather more near the surface) and have other designations by soil scientists. Clay tends to wash down into areas below the A horizon, where it ultimately accumulates. These horizons are usually called B horizons. In Mississippi's hill areas, we often find a hard, restrictive layer about 15 to 24 inches below the surface (called a fragipan). Roots cannot elongate through these special horizons, nor can water penetrate easily. To grow crops on these soils, the fragipans must be periodically shattered and broken apart.

A detailed knowledge of a soil profile figures considerably in the use of soil taxonomic systems, because soils developing under specific conditions (climate, topography, parent material, vegetation, etc.) usually develop signature profiles characteristic of, and reflecting those conditions.