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similar (displacement of the clay corner along a straight line passing
through that corner). But the inflection of the trajectories towards the
sand corner in certain very mature soils (Fig. 3.18) should make us
suspect that they evolve under the influence of the solubilization of
their clayey components and not only under the influence of their
redistribution through translocation of clay. We shall return to this
question during the study of Luvisols (Chap. 8).
Lastly, all that we have seen indicate that great transformations in
particle-size distribution occur from one horizon to another during
pedogenetic evolution. This confirms what we had said in Chapter 2
(§ 2.2.4): it is not enough that the particle-size distribution varies from
one horizon to the other for the profile to be considered as the result
of the superposition of different geological materials!
3.5 ORGANIZATION OF SOILS AT ALL SCALES
We shall now present a few attempts at summarizing the diversity of
the soils of the world, even if it is greatly simplified.
3.5.1 Soils on the World Scale
Millot (1979b) lays stress on the diversity of clay minerals that is found
on the surface of the earth as a function of the latitude (Fig. 3.19). When
weathering is nearly zero, the soil clay minerals are directly inherited
from the rock substrata. Illites and chlorites are predominant. In the
temperate zone, the clay minerals are derived from the parent material
by slight transformation. In the Mediterranean zone, silica-rich clay
minerals are found: the smectites. On the contrary, in the intertropical
zone, desilication is widespread. Kaolinite is obtained and, often,
gibbsite.
Apart from Millot's scheme, it is necessary to mention that of
Pédro (1984), which distinguished the systems of intense weathering
leading to neoformations, the system of limited weathering resulting in
reorganization of the minerals, and the one that leaves little or no residue,
especially in the high latitudes. Lastly, Chesworth (1992) separates four
pedogenetic domains at the world scale: acid trend, reduced trend,
alkaline trend and 'non-extreme soils'.
3.5.2 Soils on the Continental Scale
Erhart (1956) in his topic La Genèse des Sols en tant que Phénomène
Géologique introduced the theory he called bio-rhexistasy . It shows there
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