Which horizon is considered the topsoil, and which is considered the subsoil?

Soil has many horizons and the horizons are classified according to many factors and the composition of matter present in it such as; composition of organic matter, mineral content, and water holding capacity. There are four different layers of soil horizon classified as O, A, B, and C horizons respectively.

The soil surface containing freshly decomposing organic matters or humus is called the O horizon. The A horizon consists of both mixtures of organic as well as inorganic products. Topsoil or the top-most layer of soil mainly ranges from two to three inches deep and can vary also. So, the top layer of soil is composed of the O horizon and some parts of the A horizon.

The layer below the topsoil is called subsoil which is classified as the B horizon that accumulates mainly minerals and clay soil. Calcium carbonate depositions are also present in the B horizon.