Like air and water, soil is necessary to life on Earth. Civilizations depend on the quality of their soil to grow their food and to serve as a living filter that purifies the wastes they produce. Only a thin layer of soil, called topsoil, can adequately support plant life. While along great rivers the soil favorable to plant growth may be hundreds of feet thick, in most places it extends down only about six inches. People must protect the fertility of this thin layer by preventing erosion and by replacing the food values that crops remove from the soil.
Erosion can strip soil from unprotected land.
The replacement of such losses is very slow. Therefore nations must learn to conserve their productive soil. Formed Soils are developed from mineral and organic matter and generally contain an active population of organisms. Unlike solid rock, soils are full of pores and channels that serve to conduct air and water. Each type of soil expresses characteristics of the parent material from which it developed and reflects changes imposed by its surrounding environment.