The capital letters O,A,E,B,C, and R represent the master horizons and layers of soils. The capital letters are the base symbols to which other characters are added to complete the designations. The mineral fraction of such material is only a small percentage of the volume of the material and generally is much less than half of the weight.
Some soils consist entirely of material designated as O horizons or layers.
An O layer may be on the surface of a mineral soil or at any depth beneath the surface, if it is buried. A horizon formed by illuviation of organic material into a mineral subsoil is not an O horizon, although some horizons that formed in this manner contain much organic matter. If a surface horizon has properties of both A and E horizons but the feature emphasized is an accumulation of humified organic matter, it is designated an A horizon. In some places, as in warm arid climates, the undisturbed surface horizon is less dark than the adjacent underlying horizon and contains only small amounts of organic matter. It has a morphology distinct from the C layer, although the mineral fraction is unaltered or only slightly altered by weathering.