Soil is unconsolidated rock particles mixed with organic matter from plant decay.
Soil is vital to all life on Earth because it supports the growth of plants, which supply food and Oxygen and absorbs Carbon dioxide and Nitrogen. Soil serves as a habitat for animal life from microorganisms to small animals.
Soils can be divided into two general layers or strata: Topsoil, the topmost layer, where most plant roots, microorganisms, and other animal life are located, and Subsoil, which is deeper and often more dense and less rich in organic matter.
Water and Air are also components of most soils. Air, trapped in spaces between soil particles, and water, trapped in spaces and on the surface of particles, comprises about half of the soil by volume. Both are important to plant growth and other life in the soil profile of a particular Ecosystem.
The rock and mineral content of soil is categorized according to particle size as Sand (coarsest), Silt or Clay (finest); the ratio of these particles to a great degree determines the soil classification and characteristics.
Former soils which become buried below the effects of organisms are called paleosols.
Soil develops naturally over time through the action of plants, animals, and weathering. Soil is also affected by human habitation. People can alter soil to make it more suitable for plant growth through the addition of organic material and natural or synthetic Fertilizer, and by improving its drainage or water-retaining capacity. Human actions also can degrade soil through the depletion of nutrients, pollution, contamination, and compaction, and by increasing the rate of Erosion, which is the relocation of soil through the movement of water or wind.
However if acid is introduced into soil, e.g. by Acid rain, 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 may apply alkaline materials such as slaked lime.
Although the elements Nitrogen, potassium and Phosphorus, which are necessary for plant growth, may be abundant in soil, only a fraction of these elements may be in a chemical form which plants can use. In processes such as Nitrification and mineralisation, bacteria and other organisms convert unusable forms (such as NH4+) 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 component of soils originate in plant debris (such as fallen leaves), animal excreta, and other decomposing organic materials. These materials, when broken down, form Humus, a dark, nutrient-rich material. Chemically, humus 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. Organic material also contributes to water retention, drainage ability, and oxygenation of soil.
If oxygen enters a wet soil, because of lowered ground water table, organic matter in the soil will be broken down further by oxidation, which can lead to Subsidence. An example of this can be seen in soils in the Everglades region of Florida, which have been drained by canals for agriculture, primarily Sugar production. Originally very high in organic content, oxygenation and compaction have led to breakdown of the soil structure and nutrient content, and degradation of the soil's ability to support continued high crop yields.