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(1) In the B horizon of Planosols, there are practically no clay
coatings. More generally speaking, most authors emphasize that
the clay coatings in the B horizons of Luvisols or Planosols and
studied in thin section are not enough to explain the enrichment
in clay (Bronger 1991). Some authors feel that these coatings
disappear by
pedoturbation
, that is, integrated in the soil mass
by biological activity and shrink-swell phenomena affecting the
clays. But in many soils, these presumed transformations have
not been proved.
(2) It is a little arbitrary to distinguish two phases in argilluviation.
It gives rise to the notion that one happens before the other.
Phenomena of brunification and bleaching occur at the same
time in the same soil. This is clearly seen in thin sections cut
from the B horizons of Luvisols on granite. Micas are weathered.
A brown clayey material is formed that seems to slip between
two quartz or feldspar crystals, then the same material is com-
pletely deferrified, one mm farther in the crack. It is the same
thing in some terrace soils (Fedoroff and Rossignol 1969).
(3) On the one hand, the clay is visibly separated from iron in the
degraded surface horizons; on the other hand, results obtained
in Luvisols of le Bourbonnais (France) show that the iron/clay
ratio remains nearly constant for hundreds of E and B horizons
(Favrot
et al
. 1973; Charoui and Charoui 2004). This contradicts
the notion that only the deferrified clay migrates.
(4) In the E horizons,
papules
are often seen that are fragments of
the B horizons containing signs of clay coatings. From where do
they come?
8.2.2
New Model of Development of Luvisols
For us, a large part of the morphology of Luvisols is explained by the
fact that the E consumes (degrades) the B from above. Our point of view
is supported a priori by the idea that the mechanisms of pedogenesis
exhibit, on the earth's surface, a sort of uniqueness. Tropical soils are
affected by this process (Chaps. 5 and 6). Why would not the same thing
happen, but to a lesser degree, in the soils of the temperate regions?
Figure 8.6 summarizes the model that we shall now discuss. It is
necessary to read it in the direction of weathering, that is from the
Progressive descent of the weathering front
