Soil Landscapes (2024)

The results of the soil-forming processes are reflected in the different horizons developed in asoil profile. The soil profile extends from the surface downward to materials that are little altered by the soil-forming processes.

Most soils contain three major mineral horizons: A, B, and C. These major horizons may be further sub-divided by the use of numbers and letters that indicate important characteristics of the subdivided horizons. An example is the B2t horizon, which designates a layer within the B horizoncontaining clay translocated from the A horizon. Most soils that have not been disturbed by cultivation have a thin organic horizon on top of the mineral soil. This organic horizon is designated by the letter O.

The A horizon is the surface mineral layer. That part of the A horizon having the largest accumulation of organic matter is called the A1 horizon. The A2 horizon is the layer of maximum leaching, or eluviation, of clay and iron. The A2 horizon of many soils in Centre County is brownish because of the oxidation of iron.

The B horizon lies underneath the A horizon and is commonly called the subsoil. It is the horizon of maximum accumulation, or illuviation, of clay, iron, aluminum, or other compounds leached from the A horizon. In some soils the B horizon forms through alteration of material in place rather than by illuviation. The alteration may be the result of oxidation and reduction of iron or the weathering of minerals. The B horizon generally has blocky or prismatic structure. It normally is firmer and lighter colored than the Al horizon, but it is generally brighter colored than the C horizon. Together, the A and B horizons constitute the solum-the zone in which most of themineral and organic matter has been added, removed, transferred, or translocated through the soil-forming processes.

The C horizon is below the A and B horizons of most soils. It consists of materials that were little altered by the soil-forming processes, though the materials may be modified by weathering. Below the C horizon in some soils is an R horizon of consolidated bedrock, such as limestone, shale, or sandstone. In a few soils the C horizon is lacking, and the R horizon is directly beneath the B horizon.