Soil

Soil is a natural body consisting of layers (soil horizons) of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. It is composed of particles of broken rock that have been altered by chemical and environmental processes that include weathering and erosion. Soil differs from its parent rock due to interactions between the lithosphere, hydrosphere, atmosphere, and the biosphere. It is a mixture of mineral and organic constituents that are in solid, gaseous and aqueous states.
Soil particles pack loosely, forming a soil structure filled with pore spaces. These pores contain soil solution (liquid) and air (gas). Accordingly, soils are often treated as a three state system. Most soils have a density between 1 and 2 g/cm³. Soil is also known as earth: it is the substance from which our planet takes its name. Little of the soil composition of planet Earth is older than the Tertiary and most no older than the Pleistocene. In engineering, soil is referred to as regolith, or loose rock material.


Soil horizons

The naming of soil horizons is based on the type of material the horizons are composed of; these materials reflect the duration of the specific processes used in soil formation. They are labeled using a short hand notation of letters and numbers. They are described and classified by their color, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics, and if they have nodules or concretions.Any one soil profile does not have all the major horizons covered below; soils may have few or many horizons.
The exposure of parent material to favorable conditions produces initial soils that are suitable for plant growth. Plant growth often results in the accumulation of organic residues, the accumulated organic layer is called the O horizon. Biological organisms colonize and break down organic materials, making available nutrients that other plants and animals can live on. After sufficient time a distinctive organic surface layer forms with humus which is called the A horizon.