Before we set about changing soil, we need to know what soil is. Traditionally, soil’s relationship to the earth is compared with the rind’s relationship to an orange. The analogy is apt if you imagine an orange with a rind that is highly irregular in width, color, texture, and composition. Be it deep or shallow, red or black, sand or clay, the soil is the link between the rock shell of the earth and the living things on its surface. We are dependent on soil for life itself.
Geologists are primarily concerned with the earth’s crust—a shell of solid rock about 20 to 30 miles thick surrounding the earth. Soil scientists, on the other hand, are concerned with the thin layer of loose material that covers this shell. This layer, the soil, can be anywhere from a few inches to many feet thick.
Every solid rock, when it is exposed at the surface of the earth, slowly disintegrates into loose material through the process called weathering. Rocks are broken up into smaller particles by frost action, by the expansion and contraction caused by temperature changes, by the grinding action of streams, glaciers and wind, and by the force of large tree roots. This physical weathering is aided by chemical weathering processes, which cause the rock minerals to dissolve slowly and change by the action of water, carbon dioxide, oxygen, organic acids, and the effects of microbial activity.
So here is this rather thin layer of loose material—mineral elements from weathered rocks, dead and living organic matter (both flora and fauna), air and water—lying on top of a thick layer of solid rock and constantly undergoing change from the effects of weather, chemical processes, microbes, plants and man.
This, then, is soil. It is a complex medium in which plant roots can grow and from which they can obtain water, air, and the nutrient elements essential to plant growth.