Mature soils are arranged in a series of zones called “soil horizons”. Each horizon has a distinct texture and composition that varies with different types of soils. A cross sectional view of the horizons in a soil is called a “soil profile”.
The top layer or the surface litter layer called the “O-horizon” consists mostly of freshly-fallen and partially decomposed leaves, twigs, animal waste, fungi and other organic materials. Normally, it is brown or black.
The uppermost layer of the soil, called the “A-horizon” consists of partially decomposed organic matter (humus) and some inorganic mineral particles it is usually darker and looser then the deeper layers. The rots of most plants are found in these two upper layers. As long as these layers are anchored by vegetation, the soil stores water and releases it in a trickle throughout the year instead of in a force like a flood. These two top layers also contain a large amount of bacteria, fungi, earthworms and other small insects, which form complex food webs in the soil, help recycle soil nutrients and contribute to soil fertility .
The “B-horizon” often called the subsoil contains less organic material and fewer organisms than the A-horizon. The area below the subsoil is called the “C-horizon” and consists of weathered parent material. This parent material does not contain any organic materials. The chemical composition of the C-horizon helps to determine the pH of the soil and also influences the soil’s rate of water absorption and retention.
Soils vary in their content of clay (very fine particles) and gravel (coarse to very particles). The relative amounts of the different sizes and types of mineral articles determine the soil texture. Soils with approximately equal mixtures of clay, and sand, silt and humus are called loams.
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