Geoscience Reference
In-Depth Information
abrupt, the case of Planosols being an example; (7) no link is found
between the thickness of the E and the thicknesses of the B horizons
because one does not directly nourish the other; (8) the model indicates
that the E horizons deepen with time and this is what is observed; (9)
the irregularities in the Bh/Bs boundary can be easily explained (see
Chap. 1, Fig. 1.3); (10) and (11) one does not look for Al and Fe in an E
horizon that no longer contains these elements. Laboratory experiments
show that we can easily extract considerable Al from the B but very
little from the A, even when the latter is not completely depleted (van
der Salm
et al
. 2000). One must expect it!
Over and beyond the points mentioned above, our model allows
better understanding of certain facts, in particular:
Every time the weathering fronts form (see Figs. 6.12 and 8.7
in the corresponding chapters, and Fig. 11.2 here), Nature gives
all sorts of different profiles through a unique mechanism, here
podzolization. When only front no. 1 intervenes, we are in the
Cambisol stage. When front no. 2 enters the scene, the Entic
Podzol stage is reached. When front no. 3 makes its appearance,
the Podzol stage is found. In addition, front no. 2 can almost
catch up with front no. 1 (Humic Podzols); front no. 3 can well-
nigh join no. 2 (essentially Ferruginous Podzols) and front 3 can
almost overhaul no. 1 (Podzols that lack B horizons).
If we agree without proof that the weathering fronts penetrate
at approximately 10 m per million years, in Podzols as in other
soils, this represents only 5 cm per 5000 years in the temperate
zone! This is compatible with what we have seen: the E horizons
of Alpine Podzols 3000- or 5000-years old are 5 to 10 cm deep.
Thus the machinery we have described works very slowly. The
annual transfers of carbon, iron and aluminium are extremely
small, making lysimetric measurements and their interpretation
very difficult. On the other hand, in tropical climate, the time
periods are so long that the result of 'soil collapse' becomes
quite perceptible. In Amazonia, the Ferrallitic soil Æ Podzol
transformation has led to lowering of the soil surface by about
70 m (Dubroeucq
et al
. 1991)
.
Finally, Podzols are soils like the others: in them, weathering of
mineral materials increases from the bottom to the top whereas the
weathering agents (H
+
ions particularly) originate from the top. This
principle of generality of pedogenetic phenomena is, one would agree,
intellectually satisfying!
