Eluviation

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Eluviation downward or oblique migration of substances in suspension within the profile causing the formation of a depleted or eluvial horizon overlying an enriched or illuvial horizon. This process plays a vital role in soil formation. Generally the movement takes place in the downward direction. But in the circumstances of drying-out of the surface soil, it may result in an upward movement. Water movement through a soil is accompanied by translocation of materials from surface horizons. Materials thus removed or eluviated are deposited in the subsurface horizons or are altogether removed in drainage.

There may be two types of eluviation: chemical eluviation and mechanical eluviation. During soil formation numerous reactions occur due to continuous interplay of atmosphere, hydrosphere and lithosphere with the intimate touch of the biosphere. As a consequence, physical disintegration and chemical decomposition of rocks and minerals occur and the organic fraction of the soil is also subject to various changes. Due to these changes new chemical species are formed, some of which interact with soil constituents and are retained in the soil while some chemical species are removed from soil. In chemical eluviation decomposition products are removed in true or colloidal solution to be deposited in other horizons. On the contrary, in mechanical eluviation the finer fractions of the mineral portion of soils, especially clay, are washed down to the lower horizons. Mechanical eluviation results in the development of a soil horizon characterised by light texture. A horizon from which their finer fractions of clay are removed and deposited in the underlying horizon makes it heavy textured. Such horizons are also enriched with chemical species, eluviated from the upper horizon. 
The eluviation of substances from the lower horizon to the upper horizon results in response to the drying-out of the upper horizon. Soluble salts, dissolved in soil water are mostly transported in a mass to the upper horizon to maintain equilibrium with respect to the water content between the horizons concerned. 
The character of eluviation depends upon the climate and is also affected by local drainage conditions. For example, in free drained soil where rainfall is more than the evaporation, water moves from the surface to the level of groundwater and leaching of the profile occurs in the same way as a precipitate is washed on a filter paper. This type of eluviation is typical for humid regions and is also seen in sub-humid regions like Bangladesh. The occurrence of upward movement of materials is typical for arid and semiarid regions where the rate of evaporation is greater than the rate of precipitation. The downward and upward movement of soluble salts controls the genesis of soils through horizon differentiation. The genesis of halomorphic or salt affected soils is largely controlled by the salt movement through profile. [Sirajul Hoque] [Hoque, Sirajul  Professor of Soil, Water and Environment, Dhaka University]