Podzolic Soils - Major Soil Groups of the World - page 336

In Podzols resulting from the ageing of other soils types, some minerals
are products of an earlier weathering.
Kaolinite . In the tropical zone, kaolinite is not present in the E horizon
(Dubroeucq and Volkoff 1988). Thermodynamic calculations show
that the soil solutions, after passing through the litter, are aggressive
against all minerals (Cornu et al . 1997). Kaolinite, if it exists, is destroyed
because of the strong acidity that renders Al slightly soluble. This
broadly explains the possible differentiation of Podzols at the expense
of Ferralsols (Grimaldi 2004; Montes et al . 2007). In the temperate zone,
kaolinite is present in Podzols, in small amounts though, because these
soils are developed only on parent materials poor in fine particles.
Thermodynamic calculations show that the mineral is unstable in the A
and E horizons. It is stable in the B horizons (Macías-Vásquez et al . 1986).
Moreover, the solution of the lower horizons of Podzols is saturated with
respect to this clay mineral and therefore does not attack it (Aurousseau
et al. 1986).
Gibbsite . I n t he t ropica l zone, g ibbsite d i sappea rs f rom t he qua r t zi ferou s
bleached horizons (Dubroeucq and Volkoff 1988). In the temperate zone
too, the mineral is destroyed in Podzols (Robert et al . 1980b). Also, this
mineral has been solubilized in the laboratory by fulvic acids (Righi et
al . 1982). However, in places it sometimes gets the time to form in small
quantities in profiles on granite (Huber 1997).
Amorphous materials . In podzolizing environment, the weathering
of silicates releases gels of alumina and silica, which yield various
compounds: allophane, proto-imogolite, imogolite, gibbsite and halloysite.
These products, almost absent in the A and E horizons, form short-lived
metastable species in the B horizons (Macías-Vásquez et al . 1986).

Attack of transformed or neoformed (secondary) minerals

If mineral-organic complexes are formed in the A and E horizons,
they could migrate and precipitate in the B horizons. Details of the
mechanisms have been summarized (de Coninck 1979) :

Precipitation of weathering products

￿ the coming together of a large amount of flocculating cations
(Fe and, above all, Al, linked to weathering in situ ); this is why
scientists attach great importance in the B horizons of these soils
to the C / metals ratio, which is practically the C /( Fe + Al ) ratio;

￿ biological decarboxylation and thus destruction of the sites where
Fe and Al are held; according to Lundström et al . (2000a), this
phenomenon is more important than the preceding;