Geology
Soil Layers
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Soil is built in layers, called horizons, that differ in colour, texture, organic matter and mineral content. Near the surface, decomposed plant material forms humus, which enriches the topsoil below. Deeper subsoil usually contains less organic matter and more minerals washed down from above.
Plant roots are often most abundant near the surface, but many can grow much deeper in search of water and nutrients.
Additional Information
This model is a simplified soil profile — real soils often include additional horizons, such as parent material beneath the subsoil.