The distinct soil layers, or horizons, are organic matter, humus, topsoil, the eluviated layer, subsoil, regolith and bedrock. The main horizons are also referred to as master horizons and are often designated and recognized using the letters O, P, A, E, B, C and R, respectively.
Not all soil profiles comprise all of the listed layers; soil profiles differ based on location. Each of the horizons varies in thickness and has a set of unique characteristics that differentiate it from the other layers. The layers are identified based on color, structure, texture or chemical composition. For example, while A horizon is dark, coarse and home to earthworms and fungi, R horizon contains hard bedrock that cannot be unearthed with human hands. Identifying the soil horizons helps reveal certain characteristics about the soil that may be essential in establishing suitable land uses.