Soil profile refers to the vertical arrangement of the various soil layers from the top layer down to the parent rock or bottom layer. It is a vertical section through the soil horizons extending into the parent material or the bedrock. It describes the sections down wards through the soil which comprises of differing characteristics in terms of texture, color, mineral composition, ratio of combination of organic and inorganic matter, hardness and rate of weathering. The different layers are referred to as horizons. Soil horizon is a well-defined layer within the soil profile parallel to the local round surface. There are four main horizons namely: A horizon, B horizon, C horizon and D horizon. Each horizon has different physical and chemical properties, which result from various soil forming processes such as weathering, introduction of humus and movement of minerals.
O-HORIZON This is the top most/ surface layer of the soil comprising of organic matter. The constituents of this layer include un-decomposed litter, decomposing organic matter and humus.
A-HORIZON This is also known as the topsoil and it is rich in organic matter which organic matter accounts for the dark colour. Leaching and Eluviation may at times impoverish the topsoil.
B-HORIZON This is known as the sub soil. Nutrients removed from the A horizon through leaching and Eluviation accumulate or are deposited in this horizon. The process of plant nutrients precipitating or accumulating in this horizon is known as illuviation. This horizon may also be characterised by hard pans due to the accumulation of large quantities of clay and other nutrients.
C-HORIZON This consists of partially weathered rock, this is because weathering and other soil forming processes may not effectively operate at this depth.
D-HORIZON This consists of the solid parent rock or unweathered rock or fresh parent material. It is also known as the bedrock. It has no soil particles but has potential for future soil formation