soil horizon

soil horizon A relatively uniform soil layer which lies at any depth in the soil profile, which is parallel, or nearly so, with the soil surface, and which is differentiated from adjacent horizons above and below by contrasts in mineral or organic properties. Soil horizons are grouped primarily into O, A, B, and C horizons. O horizons (formerly known as Ao horizons) comprise organic material at the surface. A horizons are surface horizons of mixed organo-mineral composition. Where mineral matter has been lost, the A horizon is sometimes called the E (for eluviated) horizon. Where they are present, B horizons are usually located in the middle of the sequence, and are horizons into which material (mineral and organic) is deposited, thus altering the character of the horizon. C horizons are soil parent materials, weathered but not otherwise altered by pedogenic processes. The underlying unweathered material is sometimes called the D or R horizon. In addition to these, surface litter may form an L horizon, above a layer of fermented material (F horizon) and, below that, humified material (H horizon), and a mineral crust, often cemented, is sometimes called the K horizon. See PEDOGENESIS.