Soils consist of unconsolidated, natural material that supports, or is capable of supporting, plant life. The upper limit is air and the lower limit is either bedrock or the limit of biological activity. As normally defined, a soil must support plant life. The concept is expanded to include substrates that could support plant life. For various reasons, plants may be absent from areas that have well-defined soils.
Most soils have two or more identifiable horizons.
The A-horizon, the surface soil or topsoil, is a zone in which organic matter is usually being added to the mineral. It is also the zone from which both mineral and organic matter are being moved slowly downward. The next major horizon is the B-horizon, often referred to as the subsoil. The B-horizon is the zone of maximum accumulation of materials. The lowest major horizon, the R-horizon, consists of consolidated bedrock.