The four layers of soil from top to bottom are the O, A, B and C Horizons. The soils vary in color, mineral content, structure and texture; characteristics that play an important role in their function.
The O Horizon is the thin, dark top layer that is composed mainly of leaves, plants, bugs and other decomposed materials. The A Horizon just below is also known as "topsoil." Minerals and decomposed matter primarily make up this layer, and the roots of most plants are found within. Clay and mineral deposits are found in B Horizon. This layer is lighter in color and contains less organic materials than the layers above. C Horizon is made up of slightly broken-up rock and very little organic material. Because of this, plant roots are unable to penetrate this layer.