Geoscience Reference
In-Depth Information
Convex interfluve with non-cuirassed Ferralsols
Red pedological horizons
Kaolinitic
(saprolite)
altérite
Quartz vein
Stone
line
Lowland
40 to
50 m
Light-coloured sand on arène with water table;
locally peaty on the surface
Unweathered
granite
Fig. 5.8
Toposequence of soils in the equatorial zone under forest (Raunet 1982, reported
by Bertrand 1998).
Between the hillocks, the lowlands collect the waters loaded with
organic materials and remain saturated almost permanently. The rock is
decomposed and the ions evacuated. Only washed quartz sand remains
in place. Peat can overlie it. Agricultural improvement is difficult because
there is excess water in all seasons. Also, the fertility drops rapidly after
cropping.
To obtain a deposit of pure, white kaolinite, iron must be eliminated
altogether. This can happen when water charged with organic substances
percolates through the soil so that microorganisms consume the oxygen
present; the iron is then reduced and eliminated as Fe
II
(Montes
et al.
2007). This situation also occurs in Podzols (Chap. 11).
Pure kaolinite
The process of desilication of saprolites can continue to its completion,
that is, disappearance of the kaolinite and almost exclusive concentration
of aluminium oxide. For this, the environment must favour export of
silica-rich solutions. The favourable situations (Reatto
et al.
2008) are:
well-drained situations (topslopes), regions of high annual rainfall, long
periods of time. But it is also necessary for iron to be evacuated. This is
easier when the medium is temporarily saturated with water and, hence,
reducing, at the slope bottom particularly. Thus there is a contradiction!
Actually, exploitable deposits of bauxite are often pockets of materials
moved from the slopes and accumulated in depressions.
Gibbsic Ferralsol
