also soil horizon a layer of soil approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organisms present, degree of acidity or alkalinity; kinds of soil horizon include the following:

—albic horizon a mineral soil horizon from which clay and free iron oxides have been removed, or in which the oxides have been segregated to the extent that the color of the horizon is determined primarily by the color of the primary sand and silt particles, rather than by coatings on these particles

—argillic horizon a mineral soil horizon that is characterized by the illuvial accumulation of phyllosilicate clays —see illuviation

—cambic horizon a mineral soil horizon that has a texture of loamy, very fine sand or finer, has soil structure, contains some weatherable minerals, and is characterized by the alteration or removal of mineral material as indicated by mottling or gray colors, stronger chromas or redder hues than in underlying horizons, or the removal of carbonates

—illuvial horizon a soil layer or horizon in which material carried from an overlying layer has been precipitated from solution or deposited from suspension; the layer of accumulation

—A horizon a mineral horizon that formed at the surface or below an O horizon, obliterating all or much of the original rock structure and (a) characterized by an accumulation of humified organic matter intimately mixed with the mineral fraction and not dominated by properties characteristic of E horizon or B horizon, or (b) with properties resulting from cultivation, pasturing, or similar kinds of disturbance

—B horizon a soil horizon that formed below an A horizon, E horizon, or O horizon, dominated by obliteration of all or much of the original rock structure and showing one or more of the following: (a) illuvial concentration of silicate clay, iron, aluminum, humus, carbonates, gypsum, or silica, alone or in combination; (b) evidence of removal of carbonates; (c) residual concentration of sesquioxides; (d) coatings of sequioxides that make the horizon conspicuously lower in value, higher in chroma, or redder in hue than the overlying or underlying horizons without apparent illuviation of iron; (e) alteration that forms silicate clay or liberates oxides, or both, that forms granular, blocky, or prismatic structure if volume changes accompany changes in moisture content; or (f) brittleness

—C horizon a horizon or layer, excluding hard bedrock, that is little affected by pedogenic processes and lacks properties of O, A, E, or B horizons —note the material of C horizons may be either like or unlike that from which the solum presumably formed

—E horizon a mineral soil horizon in which the main feature is loss of silicate clay, iron, aluminum, or some combination of these, leaving a concentration of sand and silt particles of quartz or other resistant materials

—F layer the fragmentation layer in the forest floor, immediately below the litter layer, composed of fragmented, partially decomposed organic materials that are sufficiently well preserved to permit identification as to origin —note the term is no longer used in the US system of soil taxonomy —see Oa horizon

—H layer the humus layer of the forest floor, consisting largely of well-decomposed, amorphous organic matter —note the term is no longer used in the US system of soil taxonomy —see Oe horizon

—L layer the litter layer of the forest floor, consisting of unaltered dead remains of plants and animals —note the term is no longer used in the US system of soil taxonomy —see Oi horizon

—O horizon a soil layer dominated by organic materials occurring at the surface of the soil

—Oa horizon an organic soil horizon composed of highly decomposed organic material where rubbed fiber content averages < 1/6 of the volume —see H layer

—Oe horizon an organic soil horizon composed of organic material of intermediate decomposition in which rubbed fiber content is 1/6 to 2/5 of the volume —see F layer

—Oi horizon an organic soil horizon composed of slightly decomposed organic material in which rubbed fiber content is more than about 2/5 of the volume —see L layer This definition last updated 10/29/2008