Description and classification of soil

The relatively thin mantle of soil over the land surface of the earth is a porous material of widely varying properties. Its solid phase consists of the inorganic products of weathered rock or transported material together with the organic products of the flora and fauna that inhabit soil. Some of these products are recognizable remnants in the form of stones, sand grains, and leaf litter, but others like clay minerals and humus result from the profound chemical changes that occur in both inorganic and organic material during the process of soil formation. The resulting soil can range in texture from coarse sands to fine clayey materials and it can range in its organic content from a usual amount that is less than 5 per cent by weight to about 80 per cent in peaty soil.

These and other properties can differ greatly from place to place over the earth's surface and from top to bottom through the succession of horizons or layers that constitute the soil profile. Hence, it is useful to be able to recognize some order in the occurrence of soil materials in nature. The profile, extending from the soil surface to a depth that includes the zone explored by plant roots, forms the basis for studying soil distribution systematically. For a given set of conditions affecting its development, the profile exhibits predictable properties.