Geoscience Reference
In-Depth Information
Just as the WRB, Soil Taxonomy does not accord special status to the soils
we have studied here. It places them in Aridisols, Inceptisols, Mollisols
or Alfisols having in addition 'xeric' moisture regime and 'rhodic' colour.
It terms them, for example, Rhodoxeralfs (red Luvisols of xeric moisture
regime). In common language these are reddish Alfisols.
Rhod-ust-alfs are approximately Nitisols and Ultic Pale-xer-alfs are
Chromic Alisols.
Correlation with Soil Taxonomy
In the Americas, Nitisols are found in regions with a very pronounced
dry season: Mexico, northeastern Brazil, and south of the Tropic of
Capricorn (Volkoff 1984). These soils are called
Te r ra rox a
.
In Africa south of the Sahara, Nitisols occur in the northern part
of Cameroon and more generally in the eastern part of the continent,
because it is drier. They also exist in New Caledonia (Denis 1988).
In India, they are well represented in association with Ferralsols
in particularly humid regions. In a drier climate, they are associated
with poorly developed, yet red, soils. In the Ganges delta, dating by
thermoluminescence indicates that a rubefied soil is established in just
3000 to 6000 years (Singh
et al
. 1998). This is three or six times faster
that in the Rhône valley! But the parent material is fine and the climate
hotter and more humid.
Geographical distribution of Nitisols
Apart from the profiles described in the Rhône sequence, one can find
in the Mediterranean environment different types of soils with red
colour. This has already been pointed out. A very typical case deserves
attention (Fig. 9.10).
Other types of red soils
Hard limestone
B horizon, red, noncalcareous, argillized,
haematite-rich, with lime nodules rounded by
dissolution ( )
C horizon, yellow, goethite-rich, slightly
calcareous, with lime concretions ( )
Crack that may be more than 10-m
deep, ensuring drainage
Fig. 9.10
Structure of a pocket of red clay in karst.
