A soil profile
is a cross section through the soil
which reveals its horizons (layers). Soil generally consists of visually and texturally distinct layers, which can be summarised as follows, from top to bottom:
- A) Humus: organic matter in relatively undecomposed form. This layer tends to be dark and rich in smell and texture. Raw, semi-decomposed organic matter may be recognisable amongst its components, e.g., leaf mold, twiggy material, etc.
- B) Topsoil: well decomposed organic matter, mixed with a smaller amount of minerals.
- C) Layer of mixed decomposed organic matter and mineral content.
- D) Subsoil or mineral layers, the content of which varys according to the nature of the soil and its parent material.
- E) Bedrock or parent material, which breaks down at the upper surface due to the effects of weathering and decay. The nature of the original parent material determines the soil's composition and is itself the result of geological processes (e.g., glaciation, volcanic activity, etc., that have dominated in the region).
Although soil is divided into visually discrete horizons, constant movement occurs between them due to weathering, water movement, and the actions of soil life such as worms and bacteria, causing organic matter to move downward and mineral matter to travel upward. A soil profile can also reveal other valuable information, including evidence of compaction, water logging, root penetration, and so on.