epipedon

An epipedon is a diagnostic surface horizon or set of horizons at or near the soil surface used in soil classification to define and distinguish soil orders. The concept, developed in soil taxonomy, helps describe the uppermost part of the soil that has been altered by processes such as organic matter accumulation, weathering, or leaching. Epipedons are chosen from horizons within roughly the top 40 to 100 cm of soil, and their properties help differentiate soils in widely used classification schemes, including the USDA system and the World Reference Base.

The main types of epipedons are defined by key characteristics:

- Mollic epipedon: a thick, dark, base-rich surface horizon, usually at least 25 cm thick, with high base saturation in the upper part. It is a primary diagnostic horizon for Mollisols and is notable for its substantial organic matter and neutral to slightly alkaline pH.

- Umbric epipedon: similar in thickness and dark color to mollic but with base saturation below 50% in the surface layer, making it more leached or acidic than mollic soils.

- Ochric epipedon: a pale or light-colored horizon that does not meet mollic, umbric, or histic criteria. It is the default category for many soils lacking a distinctly dark or organic-rich surface horizon.

- Histic epipedon: an organic-rich surface horizon dominated by substantial organic matter, typical of Histosols and related organic soils.

Other epipedons, such as plaggen or anthropic variants, appear in some classification schemes for soils shaped by long-term human activity.

Significance: the presence and type of epipedon influence soil-order assignment (for example, Mollisols with a mollic epipedon; Histosols with a histic epipedon) and inform interpretations of fertility, drainage, and land use.