Agriculture Reference
In-Depth Information
Calcification
Calcification is characterised by the downward movement of Ca salts through the profile
either by leaching and precipitation, or by the eluviation of small particles from surface
horizons (see Milnes, 1992). This process is typical of arid and semi-arid environments
subject to alternating periods of wetness and dryness. Leaching is most effective for
the highly soluble salts but less so for those of lower solubility such as the salts of
the alkaline-earth elements, notably Ca and Mg. These may precipitate as calcite or
dolomite and accumulate in the soil profile to form Bk horizons or, following complete
cementation, dense petrocalcic horizons known as caliche or calcrete (Bkm horizons).
Eluviation
Eluviation is a stronger translocation process and describes the removal in suspension of
soil materials from particular layers and is most likely to occur when the soil particles
are not strongly aggregated and in soils where large macropores are present. Under these
conditions, clays, sesquioxides, silica (notably as phytoliths) and humic compounds may be
translocated downwards depending on the intensity of eluviation and climatic conditions.
Clays and other substances may be displaced down the profile to accumulate in the B
and sometimes in the C horizon. This process is characteristic of alfisols and ultisols
(
e.g.,
leached brown soils, fersiallitic or ferruginous tropical soils).
Podzolisation
Podzolisation is a comparable process in which a pronounced downwards translocation
of iron, aluminium and organic matter takes place. Clays in the upper horizons are decom-
posed under acid conditions into sesquioxides and silica by soluble organic compounds.
The breakdown products are then translocated downwards to precipitate and reform in
the B horizon. Thus, an eluvial E horizon typically overlies the illuvial accumulation of
these materials in the B horizon.
