Horizon, or soil horizon is layer of soil distinctive chemically or physically from the layers below or above. The A horizon, or topsoil, contains humus. Soil minerals are often washed down from this level into the B horizon, which is also known as the subsoil. The C horizon is the layer of rock under the subsoil. All of these horizons can be subdivided and have the following classification. 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. For article about the oriental food, see Hummus. ...

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.