Agriculture Reference
In-Depth Information
Apart from this, both iron and aluminium oxides are of particular agricultural
importance in that they have high affinities for phosphates and some trace metals which
they fix in non-exchangeable forms unavailable to plants. In addition, aluminium may
occur in amounts increasingly toxic to higher plants in soils where the pH falls below
ca.
5.5 since positive charges predominate and
ions are released in the soil solution.
Volcanic weathering products and other amorphous materials.
As discussed above,
the extremely-fine weathering products of volcanic glass are characteristic of the soils
known variously as andisols or andosols.
The aluminosilicate materials that give these soils their characteristic properties
(Maeda
et al
., 1977; Wada, 1985; Table I.5) are the X-ray 'amorphous' and poorly crys-
talline minerals allophane and imogolite, although lesser quantities of both minerals
also occur in podzols (spodosols) and other soils. The individual particles are very small
and their surface charges are dependent on the pH and ionic strength of the soil solution.
Allophane occurs in the form of small spheres and imogolite as small cylindrical or
string-like particles; because of their small sizes, these minerals have very large specific
surface areas (Table I.5).
In addition to that present in quartz and other crystalline and semi-crystalline forms,
silica may also occur in soils in an amorphous form (Drees
et al
., 1989). Amorphous
silica may either be inorganic in origin or derived from plants and sometimes animals.
Biogenic silica occurs largely as structures called phytoliths or plant opals which are
concentrations of hydrated silica and other materials that are precipitated within plant
tissues. These are returned to the soil surface in litter or through root decay. They occur
in a wide variety of shapes ranging up to
ca.
in length although most are less
than They differ considerably in solubility, depending on the presence of surface
coatings of Al and other factors and the less soluble types may accumulate in soils to
maxima of
ca.
3 % or occasionally more (Drees
et al.,
1989).
Annual Si return to the soil in dead plant parts has been estimated to range from
(Duchaufour, 1997; Alexandre
et al
., 1997). In a highly-weathered soil
supporting an equatorial rainforest, Alexandre
et al.
(1997) estimated that the rate of silica
release through phytolith dissolution was more than twice that due to silicate weathering.
Because of its relatively high solubility, Si is readily taken up by plants (as monosilicic
acid, and is susceptible to loss in drainage waters; it is also a Si source for
the formation of other minerals, including the phyllosilicate clay minerals (Duchaufour,
1997; Lucas
et al
., 1993).
The evolution and distribution of clays
The clay minerals present in any soil depend on its parent materials, its weathering
history and the amounts and differential solubilities of the elements present in the original
parent materials. Millot (1979) states that clay mineral formation may be conceived of
as a process involving the subtraction of elements in decreasing order of their solubilities
(Na >K >Ca >Mg >Fe >Si >A1) followed by the formation of new minerals from those
remaining.
The clay mineral composition of soils also varies qualitatively and quantitatively
with depth and regionally with climate and the other soil forming factors (Chapter II).
