As soils develop over time, layers (or horizons) form a soil profile.
Most soil profiles cover the earth as 2 main layers—topsoil and subsoil.
Soil horizons are the layers in the soil as you move down the soil profile. A soil profile may have soil horizons that are easy or difficult to distinguish.
Most soils exhibit 3 main horizons:
A horizon—humus-rich topsoil where nutrient, organic matter and biological activity are highest (i.e. most plant roots, earthworms, insects and micro-organisms are active). The A horizon is usually darker than other horizons because of the organic materials.
B horizon—clay-rich subsoil. This horizon is often less fertile than the topsoil but holds more moisture. It generally has a lighter colour and less biological activity than the A horizon. Texture may be heavier than the A horizon too.
C horizon—underlying weathered rock (from which the A and B horizons form).
Some soils also have an O horizonmainly consisting of plant litter which has accumulated on the soil surface.
The properties of horizons are used to distinguish between soils and determine land-use potential.
