Geoscience Reference
In-Depth Information
Other phenomena are involved in acidification. Oxidation of NH
4
+
to NO
3
-
by microorganisms supplies protons. Roots also excrete protons
and acidify the rhizosphere…
Podzols are formed on acid rocks. It is the principal minerals these rocks
contain that we shall examine here.
Quartz
has a mineral structure that makes it hardly soluble in water
even at 25° C (about 7 ppm or about 7 mg per litre). It is 5 times less
soluble at 0° C. Its solubility does not depend on pH if it is between 5
and 8. On the contrary, some silicates, especially opal and phytoliths,
weather easily. Their silica is soluble to 200 ppm at 25° C. When present
they saturate the soil solution with silica, which contributes to protecting
quartz. The latter accumulates by relative concentration. However, in
Podzols in tropical climate, quartz grains still show signs of corrosion
increasing toward the top of the profiles (Horbe
et al
. 2004).
Feldspars
are corroded then solubilized. The bases and Si are exported.
Iron and aluminium are partly retained.
Clay micas
. The evolution of clay minerals has been studied
experimentally with the help of test-minerals, especially vermiculite
(Chap. 1). The work is summarized as follows (Fig. 11.6).
Attack and evolution of primary minerals
E horizons in Podzols
Cambisols and B in Podzols
Release of
octahedral Al, then
trapping in
interlayer position,
thus formation of
aluminous
intergrades
Formation
of expandable
interstratified
mica-vermiculite
and evacuation of
free aluminium
by complexation
in situ
Vermiculite trioctahedral
test-mineral added to the
medium
(
)
Fig. 11.6
Use of test-vermiculite in soils (Ranger
et al.
1992).
When Al
3+
is trapped in the interlayer position, the CEC decreases. This
applies to soils that are not very acid and to the B horizons of Podzols
(Righi and Wilbert 1984). We may thus find 2/1/1 hydroxy-aluminous
clay minerals in contact with aluminous gels (Robert
et al
. 1980a; Dalla-
Piazza 1996). On the contrary, as soon as iron is released, it is oxidized
or hydroxylated. Thus it cannot penetrate the structure of clay minerals
(M. Robert, pers. comm.). On the other hand, in the E horizons of
Podzols, the aluminium is captured by organic matter and is eliminated.
This favours the transformation of clay minerals by expansion and the
formation of interstratified clay minerals and, sometimes, smectites of
