Soil is a natural body that consists of layers (soil horizons), composed primarily of minerals, which differ from their parent materials in their texture, structure, consistency, color, chemical, biological and other physical characteristics. The end result, soil, is the end product of the influence of the climate (temperature, precipitation), relief (slope), organisms (flora and fauna), parent materials (original minerals), temperature, and time. In engineering, soil is referred to as regolith, or loose rock material. Strictly speaking, soil is the depth of regolith that influences and has been influenced by plant roots and may range in depth from centimeters to many meters.Soil is composed of particles of broken rock (materials) that have been altered by chemical and mechanical processes that include weathering, erosion and precipitation. Soil is altered from its parent material by the interactions between the lithosphere, hydrosphere, atmosphere, and biosphere. It is a mixture of mineral and organic materials that are in solid, gaseous and aqueous states. Soil is commonly referred to as earth or dirt; technically, the term dirt should be restricted to displaced soil.Soil forms a structure filled with pore spaces that can be thought of as a mixture of solids, water and air (gas). Accordingly, soils are often treated as a three state system. Most soils have a density between 1 and 2 g/cm3. Little of the soil of planet Earth is older than the Tertiary and none older than the Pleistocene.Darkened topsoil and reddish subsoil layers are typical in some regions.On a volume basis a good quality soil is one that is 45% minerals (sand, silt, clay), 25% water, 25% air, and 5% organic material, both live and dead. The mineral and organic components are considered a constant while the percentages of water and air are the only variable parameters where the increase in one is balanced by the reduction in the other.Given time, the simple mixture of sand, silt, and clay will evolve into a soil profile that consists of two or more layers called horizons that differ in one or more properties such as texture, structure, color, porosity, consistency, and reaction. The horizons differ greatly in thickness and generally lack sharp boundaries. Mature soil profiles in temperate regions may include three master horizons A, B and C. The A and B horizons are called the solum or "true soil" as most of the chemical and biological activity that has formed soil takes place in those two profiles.The pore space of soil is shared by gasses as well as water. The aeration of the soil influences the health of the soil's flora and fauna and the emission of greenhouse gasses.Of all the factors that influence the evolution of soil, water is the most powerful due to its effect on the solution and precipitation of minerals, plant growth, the leaching of minerals from the soil profile and the transportation and deposition of the very materials of which a soil is composed.Soil colloidal particles (clay and humus) behave as a repository of nutrients and moisture, and buffer the variations of soil solution ions. Their contributions to soil nutrition are out of proportion to their part of the soil. Colloids act to store nutrients that might be leached and to release those ions in response to soil pH.Soil pH, a measure of the hydrogen ion (acid-forming) soil reactivity, is a function of the soil materials, precipitation level and plant root behavior. Soil pH affects the availability of nutrients.Most nutrients, with the exception of the lack of nitrogen in desert soils, are present in the soil but may not be available due to high or low pH. Most nutrients originate from minerals, are stored in organic material both live and dead and on colloidal particles as ions. The action of microbes on organic matter and minerals may free nutrients for use, sequester them, or cause their loss from the soil by their volitalization to gasses.