Soil is vital to the many organisms that live in it and to all animals that eat plant food.

A soil starts to form when bacteria and small plants, such as mosses, begin to grow in decomposed, weathered rock.

Humus, a dark organic material, is added when plants and animals die and rot.

Then plant roots, as well as burrowing animals, mix the contents of the new soil, keeping it porous and spongelike. This allows water, air and minerals to circulate.

Plants stabilize the soil by their root systems. It takes about 50 years for one centimeter of soil to form.

Most soils contain three distinct layers, or horizons. The A horizon (topsoil) consists of decomposed rock and humus. Horizon B (subsoil) is red or brown in color and is made up of clay and iron with little organic material. The C horizon consists of partly decomposed rock. It grades down to solid, unaltered rock.

Soil types and colors depend partly on the type of rock from which they develop and partly on the climate of the area in which they occur.

Different types of soils include chernozems (black earths), chestnut-brown soils, podsols, prairie soils and red, tropical laterites.