Geoscience Reference
In-Depth Information
A hypothesis is that the particles evolve by dissolution so that they
are attacked according to their surface-area/mass ratio. The clays are
thus preferentially affected, but the fine silts are attacked too. The silt/
sand ratio diminishes towards the top of the profile (this was more
elaborately explained in Chap. 3, Fig. 3.18). Of course, conversely, one can
think that the transport of clay partially affects the silts and therefore
the point B in the diagram is displaced to the right relative to the line
going from point E to the clay apex. But the former hypothesis has the
advantage of being consistent with the observations.
In addition, we have introduced the law of staggering of the maximum
(Chap. 3, Table 3.5). It characterizes the particle-size distribution of soils
the particles of which are subjected by simulation to solubilization
phenomena. Yet, 11 of the 12 Luvisols chosen by Jamagne (1973) in his
study of the loessial formations of northern France satisfy this rule. The
same holds true for most of the soils that we have studied on the high
weathering surfaces corresponding to sands and clays of le Bourbonnais
(Legros 1982).
8.2
DEVELOPMENT OF THE PROFILES
8.2.1
The Old Scheme and its Limitations
The concept of 'lessivage' (argilluviation) is old and very intuitive.
We find it expressed by the first soil scientists of the 19th century
(Müller, Dokuchaev followed by his student Glinka…). For these authors
and their modern disciples: when the C horizons have a clay content
intermediate between that of the A horizons and that of the B horizons,
it is so because the clay that is missing from the A is accumulated in the
B. The clay coatings found in the B seem to be irrefutable proof of the
phenomenon. The development of Luvisols is summarized as follows,
forgetting to include in the reasoning the formation of clay minerals (in
the B above all) and their degradation (chiefly in the E).
Initially, the brown clay associated with iron moves from the E
horizons, which are depleted of clay, to the B horizons that are gradually
enriched (Fig. 8.5). Increasingly differentiated soils are obtained, the
degree of evolution capable of being schematized by the ratio (
clay of
B
)/(
clay of A or E
). Starting from 1, it can reach or even exceed 3.
In the second stage, with clay illuviation continuing, the pores
of the B are slowly filled up with clay. This horizon thus becomes
A simplistic scheme
