Soils are named and classified on the basis of physical and chemical properties in their horizons (layers). "Soil Taxonomy" uses color, texture, structure, and other properties of the surface two meters deep to key the soil into a classification system to help people use soil information.
Soils and their horizons differ from one another, depending on how and when they formed. Soil scientists use five soil factors to explain how soils form and to help them predict where different soils may occur. The scientists also allow for additions and removal of soil material and for activities and changes within the soil that continue each day.
When mapping soils, a soil scientist looks for areas with similar soil-forming factors to find similar soils. The colors, texture, structure, and other properties are described. Soils with the same kind of properties are given taxonomic names. A common soil in the Midwest reflects the temperate, humid climate and native prairie vegetation with a thick, nearly black surface layer. This layer is high in organic matter from decomposing grass. It is called a "mollic epipedon." It is one of several types of surface horizons that called "epipedons." Soils in the desert commonly have an "ochric" epipedon that is light colored and low in organic matter. Subsurface horizons also are used in soil classification. Many forested areas have a subsurface horizon with an accumulation of clay called an "argillic" horizon.