Agriculture Reference
In-Depth Information
Monosiallitisation
Monosiallitisation is the process which leads to the formation and accumulation of
secondary 1:1 clay minerals, most commonly kaolinite. In this situation, the alkaline
cations from clay sheets and pan of the silica have been lost a situation typical of
ferrallitic soils (oxisols and ultisols; Soil Survey Staff, 1999).
Allitisation
Allitisation is an extreme stage of hydrolysis, rarely achieved, in which all the silica and
alkaline cations have been lost and only gibbsite
derived from the original
clay minerals and hydrated ferric oxides remain.
Apparent bisiallitisation
Where weathering is limited to slow and incomplete hydrolysis, as in cold or dry
climates, primary minerals remain largely unmodified for long periods. Under these
circumstances, the plasma comprises primary 2:1 minerals (micas) inherited from
the parent material and secondary minerals resulting from their transformation.
The formation of secondary minerals is limited to hydrated iron oxides and Clay
minerals of the 2:1 type are dominant and weathering involves a partial hydrolysis of the
primary clays through replacement of the interlayer with hydrated and ions
from the weathering solution. This leads to the formation of vermiculite and then
smectite (Pédro and Sieffermann, 1979) in an apparent bisiallitisation. Examples of this
are found in the brown soils (udalfs; Soil Survey Staff, 1999) of temperate regions
and fersiallitic soils (ustalfs; Soil Survey Staff, 1999) of semi-arid regions.
3.1.5
EFFECTS OF CLIMATE ON THE WORLD DISTRIBUTION OF THE MAJOR
WEATHERING PROCESSES
Climate is the major determinant of the weathering process through its control of
temperature and moisture, water flux and the decomposition rates of organic materials.
In extremely cold climates, weathering is dominated by physical processes while in cool
and temperate climates a limited biochemical weathering dominates. As the climate
becomes increasingly warm, a more intense geochemical weathering gradually
supervenes to become completely dominant in humid equatorial soil environments
(Duchaufour, 1997).
The coldest soils of the world occur in the cold desert of Antarctica and perhaps in
northern Greenland (Campbell and Claridge, 1992); only slightly less cold are those of
the subadjacent northern polar desert (Ugolini, 1986a). The dominant weathering
processes in these largely unvegetated environments are physical although a limited
chemical weathering of rocks and soils is evidenced by the red and brown staining that
occurs on rocks, the oxidised horizons that occur in the soils and the presence of soluble
salts derived from rock weathering. Some hydration and transformation of the 2:1 clays
present has also been reported (Campbell and Claridge, 1992). The major chemical
weathering processes in these environments are salination (and sometimes alkalinisa-
tion), calcification and oxidation (Ugolini, 1986ab).
