Definition - What does Soil Horizons mean?
Soil horizons are layers within a soil profile that are parallel to the soil surface. Soil horizons are characterized by differences in color, texture, structure, consistence and coatings. Soil horizons are designated letter codes in order to identify the type of horizon. Topsoil is designated as Horizon A. Subsoil horizons are designated as Horizon B.
Safeopedia explains Soil Horizons
The five soil horizons are O, A, E, B and C. R is used to designate bedrock. The horizons do not follow a set order. Some soil profiles may contain all five horizons, while others may only contain two horizons. The A horizon, commonly referred to as topsoil, is a mineral rich horizon. A B horizon will typically accumulate clay, soluble salts, and iron.