(1) The loose top layer of the Earth, usually consisting of disintegrated rock with an admixture of organic matter and soluble salts, dynamic in nature, and serving as a natural medium for the growth of land plants. It differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics. It usually only extends to the depth important for plant growth, the depth to root growth limiting material or 60 inch whichever is encountered first. Spatially, engineers recognize that there is an ungeneralizable, infinite variety of soils, each with slightly differing physical characteristics, the regolith. Agricultural soil scientist believes that "soils" can be mapped as significantly homogeneous, discrete units, i.e., soil types. It includes the surface soil (horizon A), the subsoil (horizon B) and the upper portion of the substratum (horizon C) to the extent that it is penetrated by plant and tree roots. The average soil is composed of 45% mineral, 25% air, 25%- water and 5% vegetation. (2) Surface layer of the Earth, ranging in thickness from a few inches to several feet, composed of finely divided rock debris mixed with decomposing vegetative and animal matter which is capable of supporting plant growth. (3) The mineral substrate plus the living and dead organic material within it. The biotope may be, in part, soil. Soil may be geochemically separated through time.