soil horizon

soil horizon A relatively uniform layer of soil, more or less parallel to the soil surface, which is physically, chemically, and/or biologically distinguishable from the layers above and below it. 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 they 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.