A soil profile is a vertical section of soil like the diagram above. It allows you to examine the structure of soil. A soil profile is divided into layers called horizons. The main soil horizons are A, B, C and D.
Most important for plant growth, the A and B horizons are the top two layers of the soil. The A horizon is where there is most soil life and is sometimes called topsoil. Depending on the soil, the A horizon can be further divided into A1, A2 or Ao (organic). The B horizon is where clays and materials washed down from the A horizon accumulate. This is sometimes called subsoil.
The C Horizon consists of weathering rock.
The D horizon is bedrock. This is rock which has weathered to produce the soil you see above it (unless the soil has been deposited from elsewhere eg floodplains contain soil that has been carried downstream in water and then deposited as the flood recedes).
Generic soil structure image
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