Soil is an important natural resource that covers much of the earth's land surface. Most life on earth depends upon the soil as a direct or indirect source of food. Plants are rooted in the soil and obtain nutrients (nourishing substances) from it. Animals get nutrients from plants or from animals that eat plants. Certain microbes in the soil cause dead organisms to decay, which helps return nutrients to the soil. In addition, many kinds of animals find shelter in the soil. Soil contains mineral and organic particles, other plant and animal matter, and air and water. The contents of soil change constantly. There are many kinds of soils, and each has certain characteristics, including color and composition. The kind of soil in an area helps determine how well crops grow there.

Composition of soils How Soil is Formed Characteristics of soils How soils are classified Soil conservation

Soil forms slowly and is destroyed easily, and so it must be conserved so it can continue to support life.

Soil scientists, called pedologists, use the term polypedons for the bodies of individual kinds of soil in a geographic area. Polypedons can be indefinitely large, but some have a surface area of only about 10.8 square feet (1 square meter). Some polypedons measure less than 5 inches (13 centimeters) deep. Others are more than 4 feet (1.2 meters) deep.

Composition of soils

The mineral and organic particles in soil are called soil particles. Water and air occupy the spaces between the particles. Plants and animals live in these pore spaces. Plant roots also grow through the pore spaces.

Minerals supply nutrients to green plants. Particles called sands, silts, and clays make up most of the mineral content of soils.

Sands and silts are particles of such minerals as quartz and feldspars. Clays consist of illite, kaolin, micas, vermiculite, and other minerals. Trace amounts of many minerals add nutrients, including calcium, phosphorus, and potassium, to the soil. Most soils are called mineral soils because more than 80 per cent of their soil particles are minerals.

Plant and animal matter consists of organic material in various stages of decay. Many organisms also live in the soil. These soil organisms include plant roots, microbes, and such animals as worms, insects, and small mammals. Bacteria, fungi, and other microbes decompose (break down) dead plants and animals. Many soil organisms help mineral and organic particles aggregate (come together) and form clumps of soil. Roots, burrowing animals, and natural weathering break apart large clumps of soil.

Decaying organic material releases nutrients into the soil. In addition, some organic material combines with mineral particles. Other decaying material forms organic soil particles called humus. Most humus is black or dark brown, and it holds large amounts of water. Only 6 to 12 per cent of the volume of particles in most mineral soils is organic.