At the time of soil formation process so many physical, chemical and biological activities occur simultaneously and ultimately after a long process of hundreds of years the soil having characteristic layers is formed. The vertical appearance of different layers parallel to the soil surface whose physical, chemical, biological characteristics differ from the layer above and below is called soil horizon. Soil horizons are classified according to their vertical sequence of distinct layers. Mostly the boundaries of soil horizons are zone of transition rather than distinct lines of division. The width of each horizon i.e. depth of the upper and lower boundaries of each horizon provide information of the soil forming processes that have formed the soil. Most of the horizons do not have a constant depth.
For describing and interpreting soil horizons in the field the master horizon codes that are assigned to the various soil horizons are H, O, A, E, B, C and R. ‘H’ horizon is the topmost layer of soil that is underwater and consists of undecomposed or partially decomposed organic material. ‘O’ horizon (humus or organic horizon) is composed of decomposed organic matter in wetlands and partially decomposed organic matter and litter at the surface of terrestrial soil. ‘A’ horizon (the mineral horizon or top soil) consists of decomposed organic matter and inorganic minerals. Living creatures in soil are also chiefly found in this horizon. ‘E’ horizon (eluviated horizon) is formed when compounds of iron, aluminium etc and organic matter are leached down from this layer. ‘B’ horizon (sub soil) consists of the iron, aluminium and humic compounds and clay leached down from A and E horizon. ‘C’ horizon (parent material) is the weathered zone of parent rock beneath and ‘R’ horizon is the bedrock from which the weathered inorganic material is coming from. Sometimes the top soil horizons are further categorized into sub-types if any distinct features of differentiation are found. Soil horizons can be seen by digging the soil deep down.