Soil

Soil is different things to different people. To most of us it is something to be washed from clothes and swept out the door. To the geologist soil is the unconsolidated, weathered part of the earth’s mantle, an insignifi cant, small fraction of its total volume. To the civil engineer it is the medium for the support of structures and a construction material. But to the agriculturist, soil is that part of the earth’s crust in which the roots of plants grow. It is a vital, living component of the envi-ronment–a component that can be manipulated to affect crop performance. When soil is misused, crops become less productive; when handled with due consideration for its biological and physical nature, it can continue to yield crops throughout countless generations of cultivation and use. Soil has 3 primary functions in sustaining plant life: It supplies mineral elements, serving both as a medium of exchange and as a place of stor-1. age. It supplies water and serves as a storage reservoir.2. It serves as a medium within which the roots of terrestrial plants, as well as those of many 3. aquatic plants, anchor themselves. One might ask whether soil is really necessary for plant growth. Hydroponics—the soil-less cultivation of plants with nutrient solutions alone or with sand or gravel—is practiced today on a limited scale. The United States Armed Services use hydroponic gardens to provide fresh vegetables for personnel on isolated islands in the Pacifi c Ocean where soil is not available. Even though it is possible to maintain a limited agriculture without soil, the massive production of plant materials that the world requires cannot be accomplished economically without soil under present economic conditions. SOIL SYSTEMS For the purposes of crop production, soil must be considered to be a delicate balance of inter-woven and interacting systems: Inorganic minerals1. Organic matter2