Soil is the layer of minerals and organic matter, in thickness from from centimeters to a meter or more, on the land surface. Its main components are mineral matter, organic matter, moisture, and air. Soils differ in the ratio of these components.
Minerals in soil are obtained from a variety of sources, but the process which delivers the bulk is weathering of rocks. Weathering is the actions of wind, rain, ice, sunlight, and biological processes on rocks that break them down into smaller particles.
Weathering also releases ions such as K+ and Mg2+ in to the soil solution. Some of these ions are taken up by plants, but the majority not left in solution are absorbed through ion exchange by clays such as montmorillonite. When the level of ions is low in the soil an equilibrium process forces ions back into solution, where they can be used by plants.
However if acid is introduced into soil, hydrogen ions bind in preference to clays, forcing ions out where they can be washed away during rain. Acidity also encourages the weathering of clays, releasing toxic aluminium ions (of which clays are composed) into the solution. To stop this occurring farmers apply alkalic materials such as slaked-lime.
Although there exists plenty of elements such as nitrogen, potassium and phosphorus necessary for plant growth in soil, very little of this is in a form which plants can use. In processes such as nitrification and mineralisation, bacteria and other organisms convert unusable forms (such as NH4+) in to usable forms (such as NO3-). The raw products are initially present as gases in the atmosphere. Processes such as the nitrogen cycle and carbon cycle continually exchange nutrients between the soil and atmosphere.
The organic store in soil is made up of plant debris, animal excreta and other decomposing materials. A lot of the carbon compounds react to form humus, which is composed of very large molecules including esters of carboxylic acid, phenolic compounds and derivatives of benzene. Organic material in soil provides nutrients necessary for plant growth.