Each layer is distinguished from the other by its colour and the size of particles. These layers are called horizons, and their arrangement is called the soil profile. In a typical mature soil there are four horizons, designated (from the surface downward) as A, B, C, and R.
In some soil the A and B horizons may have finer subdivisions.
(i) Top Soil:
The A horizon is the layer also called the topsoil. It is usually distinguished from the other horizons by its colour, texture, and structure and is rich in humus and under composed organic matter.
There is an abundance of plant root and living organisms. The colour of the top soil is determined by its drainage.
(ii) Sub Soil:
The B horizons, also called subsoil, usually is finer in texture than the A horizon and has less humus and undecomposed organic matter and fewer plant roots and living organisms.
(iii) Bed Rock:
The C horizon consists of parent material that has been changed very little by soil forming processes.
Some soil never develop a B horizon. The R horizon consists of unweathered bedrock.