Soil is the biologically active, porous medium that has developed within the uppermost layer of Earth’s crust. Soil is one among the principal substrata of life on Earth, serving as a reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant within the cycling of carbon and other elements through the worldwide ecosystem. it’s evolved through weathering processes driven by biological, climatic, geologic, and topographic influences.
Since the increase of agriculture and forestry within the 8th millennium BCE, there has also arisen by necessity a practical awareness of soils and their management. within the 18th and 19th centuries, the economic Revolution brought increasing pressure on the soil to supply raw materials demanded by commerce, while the event of quantitative science offered new opportunities for improved soil management. The study of soil as a separate science began about an equivalent time with systematic investigations of drugs that enhance plant growth. this first inquiry has expanded to an understanding of soils as complex, dynamic, biogeochemical systems that are vital to the life cycles of terrestrial vegetation and soil-inhabiting organisms and by extension to humanity also.
Soils differ widely in their properties due to geologic and climatic variation over distance and time. Even an easy property, like the soil thickness, can range from a couple of centimeters to several meters, counting on the intensity and duration of weathering, episodes of soil deposition and erosion, and therefore the patterns of landscape evolution. Nevertheless, in spite of this variability, soils have a singular structural characteristic that distinguishes them from mere earth materials and is a basis for his or her classification: a vertical sequence of layers produced by the combined actions of percolating waters and living organisms.
These layers are called horizons, and therefore the full vertical sequence of horizons constitutes the profile. Soil horizons are defined by features that reflect soil-forming processes. as an example, the uppermost soil layer is termed the A horizon. this is often a weathered layer that contains an accumulation of humus and microbial biomass that’s mixed with small-grained minerals to make aggregate structures.
Below A lies the B horizon. In mature soils, this layer is characterized by an accumulation of clay that has either been deposited out of percolating waters or precipitated by chemical processes involving dissolved products of weathering. Clay endows B horizons with an array of diverse structural features formed from small clay particles which will be linked together in various configurations because the horizon evolves.
Below the A and B horizons is that the C horizon, a zone of little or no humus accumulation or soil structure development. The C horizon often consists of unconsolidated parent material from which the A and B horizons have formed. It lacks the characteristic features of the A and B horizons and should be either relatively unweathered or deeply weathered. At some depth below the A, B, and C horizons lie consolidated rock, which makes up the R horizon.
These simple letter designations are supplemented in two ways (see the table of horizon letter designations). First, two additional horizons are defined. Litter and decomposed organic matter that typically lies exposed on the land surface above the A horizon is given the designation O horizon, whereas the layer immediately below an A horizon that has been extensively leached (that is, slowly washed of certain contents by the action of percolating water) is given the separate designation E horizon or zone of eluviation. the event of E horizons is favored by high rainfall and sandy parent material, two factors that help to make sure extensive water percolation. The solid particles lost through leaching are deposited within the B horizon, which then is often considered a zone of illuviation.