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A distinctive layer within a soil which differs chemically or physically from the layers below or above. The A horizon or topsoil contains humus. Often soil minerals are washed downwards from this layer. This material then tends to accumulate in the B horizon or subsoil. The C horizon is the unconsolidated rock below the soil. These three basic horizons may be further subdivided. Thus, Ah horizons are found under uncultivated land, Ahp horizons are under cultivated land, and Apg horizons are on gleyed land. The B horizons are also subdivided by means of suffixes: Bf horizons have a thin iron pan, Bg horizons are gleyed, Bh horizons have humic accumulations, Box horizons have a residual accumulation of sesquioxides, and Bs horizons are areas of sesquioxide accumulation. Bt horizons contain clay minerals and Bw horizons do not qualify as any of the above. Bx horizons or fragipans contain a dense but brittle layer caused by compaction. C horizons are also subdivided: Cu horizons show little evidence of gleying, salt accumulation, or fragipan; Cr horizons are too dense for root penetration; and Cg horizons are gleyed. Additional suffixes may be used. Some soil scientists use the term D horizon for the consolidated parent rock.

In addition to these soil horizons, other layers are distinguished. Thus, the layer of plant material on the soil surface is classified as: the L horizon (fresh litter); the F horizon (decomposing litter); the H horizon (well decomposed litter); and the O horizon (peaty). A leached A horizon is termed an E horizon or eluviated horizon.

Subjects: Earth Sciences and Geography.

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