Geoscience Reference
In-Depth Information
less and less permeable. In seasons of heavy rain, water saturates the
entire profile. Therefore, atmospheric oxygen does not penetrate. What
oxygen is present in the soil is rapidly consumed by biological activity.
Anoxia appears and the environment becomes reducing. Iron is reduced,
solubilized and separated from the clay.
Clay content, %
A
E
Losses
Horizon
depleted of clay
Horizon
enriched in clay
B
Gains
C
Clay-content
curve
Fig. 8.5
Classic modelling of the differentiation of a Luvisol. The white arrows represent the
presumed translocation of clay.
The beautiful brown colour of the B is totally lost. The horizon
exhibits redoxic features (Chap. 12). This corresponds to a mixture of
brown zones (where iron is oxidized), grey, blue or green zones (where
iron is reduced) and white zones (from where it has been eliminated).
In such a clayey medium with low porosity, the circulation of water is
restricted. Hence the iron does not move too far: it is precipitated in the
form of ferruginous concretions.
Above the B, water forms a perched water table during the wet
seasons. Iron is reduced here also and separated from the clay. It
precipitates as concretions or leaves with the water. The horizon is
bleached. It is thus composed of deferrified clay and residual quartzose
silts. We have seen earlier that this material appears mobile and seems
to penetrate the B at the level of the light-coloured tongues, forming a
second phase of clay illuviation (de Coninck
et al
. 1976). At the end of
development, the B having been completely filled up, the circulation of
water becomes totally lateral. The irregularities of the clayey layer are
smoothed out. The boundary between the depleted horizon and the
enriched horizon becomes abrupt. This is the
Planosol
stage.
The scheme does not take into account all the observations.
Limitations of the scheme
