Soil Horizons

Soil forms very slowly, sometimes over hundreds or thousands of years. Under the action of climate, erosion, plants, and microorganisms, the bedrock cracks and the soil is enriched with humus. The soil gradually thickens to form different horizons.

The number of horizons, their thickness, colour, and composition vary according to the nature of the rocks that make up the soil. This number also varies according to climatic conditions, the plants and other living organisms present, the age, and topography. By studying the soil profile, we can trace the events that led to its formation. In mature soil, four main horizons can be identified. According to convention, these horizons are designated by the letters O, A, B, and C, from the surface to the bedrock.

​Horizon O: This is the surface layer composed of plant debris and humus, which is called litter or organic layer. Humus is rich in nutrients since decomposers break down debris. These nutrients are carried to the lower horizons by rainwater.	​ ​ ​ ​ source
​Horizon A: It is a layer made up of a mixture of humus and minerals. This mixture is called arable land and it is generally dark coloured. As it is rich in organic matter, this layer is essential for the growth of plants since it is very fertile. It is ventilated by burrowing animals and is very prone to erosion.
​Horizon B: This layer is very poor in humus, but it is very rich in mineral elements such as iron oxides and silicates. It is often paler in colour than the A horizon or even reddish. This is where debris from the upper horizons accumulates.
​Horizon C: We note the absence of organic matter in this layer which is only composed of altered bedrock and which is fragmented by physical and chemical factors. It can be sandy, clay-like, or hard.