Agriculture Reference
In-Depth Information
(alteroplasmation), or secondary crystallisation following the precipitation or floccula-
tion of solutes and colloids (pedoplasmation) (Flach
et al.,
1968). This plasma mainly
comprises clays and other minerals of colloidal size which may subsequently be redis-
tributed during soil formation.
Two main forms of weathering have been defined (Pédro, 1983):
3.1.4.1
Aplasmogenic weathering
Aplasmogenic, or congruent weathering occurs in situations where no plasma is formed
because of the complete dissolution of the alterable minerals and the absence of
secondary minerals from the weathered substrate. This type of weathering pattern is
characteristic of soluble salt minerals such as halite and gypsum which dissolve into
their component ions. Calcite dissolves in the same way in the reversible reaction
(Birkeland, 1984):
Further examples include the acidolysis and acido-complexolysis that occurs in
podzols, alkalinolysis by sodium carbonate in certain alkaline soils (solonetz,
Duchaufour, 1997), or attacks on minerals by acidic cations at the interface between dry
and flooded parts of the soil, ferrolysis. This last process is a special case of salinolysis
by ferrous cations in soils affected by intermittent waterlogging; it leads to the loss of
bases, acidification and clay mineral destruction, as occurs in planosols (FAO-UNESCO,
1990) or argialbolls (Soil Survey Staff, 1999).
3.1.4.2
Plasmogenic weathering
Plasmogenic, or incongruent weathering is characterised by the formation of secondary
minerals which are precipitated to comprise the plasma of the weathering materials.
Two forms may be distinguished, depending whether weathering is complete or partial.
If weathering is complete, no primary minerals remain and the plasma may be
composed entirely of secondary minerals. This process is particularly favoured in humid,
moist conditions where hydrolysis is complete, and the plasma comprises neoformed clay
minerals, ferric and aluminium oxides and oxy-hydroxides (oxisols, Soil Survey Staff,
1999). Three separate stages may be defined, depending on the intensity of hydrolysis:
Bisiallitisation
Bisiallitisation occurs where clay minerals of the 2:1 type (Section I.1.1.1.2) are formed
secondarily. This occurs, for example, in vertisols where smectites form under conditions
of impeded drainage. In these soils, silica is retained although partial dealkalinisation
(
i.e.,
removal of alkaline cations associated with the clay sheets) occurs.
