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On the plateau, the cultivated soils can be ameliorated by general
drainage. On the slope, the boggy patches can be reduced by catchdrain
ditches judiciously disposed diagonally. In the hollows, the wetlands
may be left as such, if they have heritage value.
Perched water table
Alluvial plain with
water table and
occasional flooding
from streams
(Gleysols)
Slope with lateral flow of water:
appearance of boggy patches
and rushes on the
footslope when the
substratum is very close
to the surface
A
B
C
Plateau where the soils have
a clay B leading to
saturation with water in the
rainy season (Stagnosols)
Stream
Impermeable
substratum
Water table
Diagram: J.P. Party
Fig. 12.9 Typical organization of hydromorphy in Western Europe, in a valley-slope-plateau
system with impermeable substratum. Diagram : J.P. Party.
In the dry Mediterranean environment, we could believe that soils
with redoxic features are absent. However, some depressions in which
water accumulates are reducing in summer because the oxygen demand
is high and because oxygen additions are small (no rain; it is rainwater
that will carry oxygen) (Tassinari 1998).
Soils affected by excess water are often identified by the specialized
plants associated with them. But these plants vary with region.
12.4.2 Specific Functions
Many studies on Gleysols show that oxygen can disappear from the soil
in a few hours or a few days after a flooding (Ponnamperuma 1972).
The experiments of Vizier (1970), on the contrary, suggest that at least
50-60 days are necessary for conditions of strong anaerobiosis to be
established. Many factors lead to these differences:
Establishment of anaerobiosis and return to aerobiosis
￿ Biological activity. If it is high, oxygen is rapidly consumed;
￿ The type of water. Rainwater is loaded with oxygen. Thus
saturation of the soil by rain is not enough to result in anoxia if
the saturation does not last long and if it occurs during a period
of low biological activity. This is what happens in winter in the
Mediterranean region;
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