Soils are one of Earth’s essential natural resources, yet they are often taken for granted. Most people do not realize that soils are a living, breathing world supporting nearly all terrestrial life. Soils and the functions they play within an ecosystem vary greatly from one location to another as a result of many factors, including differences in climate, the animal and plant life living on them, the soil’s parent material, the position of the soil on the landscape, and the age of the soil.

Soils are composed of four main components:

  • Mineral particles of different sizes.
  • Organic materials from the remains of dead plants and animals.
  • Water that fills open pore spaces.
  • Air that fills open pore spaces.

The use and function of a soil depends on the amount of each component. For example, a good soil for growing agricultural plants has about 45% minerals, 5% organic matter, 25% air, and 25% water. Plants that live in wetlands require more water and less air. Soils used as raw material for bricks need to be completely free of organic matter.

Each area of soil on a landscape has unique characteristics. A vertical section at one location is called a soil profile. These layers are known as horizons. Soil horizons can be as thin as a few millimeters or thicker than a meter. Individual horizons are identified by the properties they contain that are different from the horizons above and below them. Some soil horizons are formed as a result of the weathering of minerals and decomposition of organic materials that move down the soil profile over time. This movement, called illuviation, influences the horizon’s composition and properties. Other horizons may be formed by the disturbance of the soil profile from erosion, deposition, or biological activity. Soils may also have been altered by human activity. For example, builders compact soil, change its composition, move soil from one location to another, or replace horizons in a different order from their original formation.