Geoscience Reference
In-Depth Information
practically no bases (Na, K, Ca, Mg) and no corresponding sol-
uble salts (especially carbonates);
pH in NaF > 9.0, indicating the presence of free aluminium.
Furthermore, the corresponding profiles have, in general, the
following characteristics:
high biological activity (there are 2000 termite species in Africa);
contrary to a widespread notion, earthworms are abundant in
tropical soils if the environment is not too wet or too dry;
low or no humus even under forest (except in cold and/or water-
saturated environments); of course the forest is very productive
and restores considerable amounts of organic matter to the soil
every year, but it is very rapidly mineralized because of the high
temperature, humid conditions and high biological activity;
great depth.
5.1.2
Accumulations of Iron and Aluminium
Table 5.1 presents the different forms of accumulation found in tropical
soils.
Forms
Table 5.1
Forms of accumulation of iron and aluminium in tropical soils.
Diffuse
Not separated; identified by the colour (Fe) or by electron microprobe
Nodules
Hardened local accumulations without specific internal structure
Concretions
Hardened local accumulations with concentric internal structure
Pedotubules
Tubular forms caused by filling of termite holes, root channels, etc.
Cuirass
Completely indurated layer in which accumulation of Fe and Al is
predominant; cuirasses may be massive, soft enough to be cut by a
spade, lamellar, pisolitic…
Nodules and concretions are sometimes grouped under the term
glaebules
.
Table 5.2 brings together the oxides and hydroxides of iron and aluminium
found in Ferralsols. Transformations are possible in accordance with
drying (arrows).
Composition
