What is Soil?

soil horizonsWe have a working knowledge of what is and what is not soil. We know it as the earthy material that we learned to call dirt when we played in it as children. We marvel at this seemingly lifeless material that gives life to plants. We understand that it is the most basic of building materials and the foundation on which we build structures. Yet, whatever this stuff is, it nearly defies formal definition.

According to the U.S. Department of Agriculture, soil is the collective term for "natural bodies, made up of mineral and organic materials, that cover much of the Earth's surface, contain living matter, and can support vegetation out of doors. Soils have in places been changed by human activity. The upper limit of soil is air or shallow water."' The lower limit of soil is more difficult to define, but it generally coincides with the common rooting depth of native perennial plants.

Soils do not cover all of the earth's land. Non-soil land surfaces, which will not grow plants, include the ice lands of polar and high-elevation regions, recent hard lava flows, salt flats, bare rock mountain slopes and ridges, and areas of moving dunes. Engineers generally ignore the biological component of soil and consider soil to be material that can be excavated with a shovel or compacted into roadbeds or other support base. More formally, engineers may consider soil as "rock particles and minerals derived from preexisting rocks.