Soil is found in the top layers of regolith, the unconsolidated (uncompacted) matter comprised of soil, sediment, and portions of bedrock that form the outer crustal layer of the Earth's surface. Soil includes varying amounts of organic matter mainly derived from plants and animal decay.

Because soil is a superficial layer, it can be highly variable and has a composition that can be readily modified by weather (e.g., rainfall).

Soil is usually found in stratified layers (i.e. a layer of black soil over subsurface layers of sand and/or clay. Although different for each geographic area, geologists use a generic soil model from which to describe unique area differences.

A soil horizon is a coherent layer of soil—similar in characteristics such as composition, texture, and color—that define the horizon from other soil types. Geologists construct soil profiles of...