The term 'horizon' is used by soil scientists for each of the distinctive layers that occur in a soil. Each soil type has at least one, usually three or four different horizons and these are described by soil scientists when seeking to classify soils.
You will see these distinctive layers whenever you are able to see a cutting down through a soil. The topmost horizon is the 'A' horizon and, as we have noted above, is usually the darkest layer of the soil because it contains most of the organic matter. Here, remnants of plants and even animals are incorporated into the soil. The numerous organisms in this soil horizon are responsible for breaking down these additions of plants and animals and releasing the nutrients they contain to become available for the next generation of plants and animals. In the A horizon the most visible plant remains, e.g. leaves, roots, are nearest the surface. Where the breakdown of plant remains is slow, perhaps because of acid conditions, there may be distinct layers of recognisable organic remains lying at or near the surface. This is often the case, for instance in a pine forest and here it is possible to trace the various stages in the breakdown of the plant and animal remains that occur on the surface. Where the soils are used in cultivation, as in the garden or the farmers ploughed field, the organic matter is all mixed up in the topsoil and there are usually no distinct organic layers within the topsoil, and the horizon has a more or less uniform brown colour.
The B horizon is the layer beneath the A horizon, and is part of the subsoil. The B horizon is usually quite different to the A horizon in colour and in structure (the architecture of the horizon), whereas the A horizon tends to be made up of small aggregates, the B horizon often has a coarser structure, is more varied in colour and often has significant features that can be used to classify soils. These features are the result of the interaction of soil forming processes such as weathering, gleying, podzolisation, calcification. Soils in different parts of the landscape will be subject to one or more of these processes. There can be subdivisions of the B horizon according to the processes that have led to the formation of the horizon. The nature of the B horizon is key to classifying the soil.
The C horizon lies below the B horizon. It is the intermediate horizon between the more or less unaltered sediment or rock below and the B horizon above. It is often possible to detect from which rocks or sediments the soil has formed by the appearance and content of the C horizon. If the C horizon has developed from the underlying rock then it is also likely to contain numerous fragments of these rocks. The C horizon contains fewer organisms that the horizons above and it contains very little organic matter, except where roots have penetrated the horizon.