The superficial layer that covers large areas of the earth's crust. It consists of mineral particles, decaying and decayed organic material, living organisms, air, and water. It is the medium in which plants grow, supporting them and supplying them with nutrients, and is also a habitat for numerous animals and microorganisms.
The mineral content of the soil is derived from the mechanical or chemical weathering of exposed rocks. A soil may be formed from the underlying bedrock (sedentary soil) or it may result from the deposition of mineral particles transported by such agents as water or wind, resulting in a mixture of particles of different origins (sedimentary soil). The proportions of the various sizes of mineral particles (silt, clay, sand, gravel, etc.) have a profound effect on the *soil texture
. This is further influenced by the amount of organic matter present, most of which is concentrated in the top 230-300 mm of soil. Living organisms affect the soil in a number of ways. The larger animals, notably earthworms, mix and aerate the soil and billions of microorganisms feed on and decompose the dead organic matter, thus making nutrients available to the plants. The water in the soil contains dissolved mineral substances and gases that can be taken up by plant roots. The level at which the soil becomes saturated with water (the water table) rises and falls depending on the amount of precipitation. Its level affects soil aeration. The air present in the spaces between the soil particles is generally richer in carbon dioxide and poorer in oxygen than atmospheric air. There are a number of different methods used to classify soil. Generally soil can be divided into three main groups: zonal soils, where the soil type reflects the prevailing climatic conditions; intra-zonal soils, where some other factor, such as the nature of the parent rock, has more influence on soil development; and azonal soils, which are immature newly formed soils, e.g. alluvial soils. Zonal soils may be further subdivided into *pedalfers
. Intra-zonal soils can be subdivided into hydro-morphic soils (with excessive moisture), halomorphic soils (with a high salt content), and calcimorphic soils (rich in lime from the parent rock). Soil colour may be used as an indication of soil composition, amount of aeration, drainage, etc. For example, badly aerated soils are often bluish grey whereas well aerated soils are reddish because of the high concentration of iron oxides. See also soil structure
, soil profile