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rubefaction takes place if the climate is favourable. It is then
seen that decarbonatation, which produces a thin, yellow horizon
named b , closely precedes rubefaction.
In India, where one finds the entire range of red soils with smectites,
illites or kaolinites at the same time, it has been shown that Ferralsols
occur when the average climatic drainage becomes positive (Bourgeon
and Pédro 1992). Drainage here is the quantity S(P monthly - PET monthly ) -
100, evaluated to a depth of one metre. The values are expressed in mm,
and 100 approximately represents the quantity of water that the soil can
retain at the end of the rainy season and is, therefore, subtracted from
the discharge. On the other hand, the soils often retain a calcareous pan
at the base when the drainage is never positive; this is the same in the
Mediterranean regions.
Role of altitude
Specific role of drainage
In the Mediterranean limestone mountains, red soils can occur at 2000-m
altitude if the climate is dry, for example in Morocco (Michalet 1991) or
in Lebanon (Darwish and Zurayk 1997). Further north, in Spain, they are
seen even to 1400 m. But above 800 m, the soil profiles present a yellow
horizon (Vaudour 1979). The horizon sequence is E/B red /B yellow /R. In
China, the haematitic soils (Acrisols) of the southwest of the country are
replaced, in the mountains, by yellow goethitic soils (Felix-Henningsen
et al. 1989). In other words, goethite indicates a more humid pedoclimate
linked to altitude. At the same time, in Lebanon, when altitude increases,
smectites gradually give place to kaolinite, but this does not allow us
to decide between neoformation and direct inheritance from the parent
material as the mechanism responsible (Darwish and Zurayk 1997).
9.2.3 Role of Vegetation
In Mediterranean climate, plants must withstand drought in summer
and water-saturation in winter. The typical vegetation that corresponds
to the red soils on limestone is scrub, the symbolic species of which is
the holm oak, Quercus ilex (Bech et al . 1997). This is an evergreen woody
species that provides scarcely any litter and little humus. Moreover,
researchers have noted that the soils rich in organic matter are less red in
colour. Apart from the masking effect related to the impregnation of the
profile with carbon, the oxalic acid produced by certain fungi is capable
of leaching iron. This process has even been suggested, on industrial
scale, for purifying and bleaching kaolinite for use in manufacture of
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