Geoscience Reference
In-Depth Information
￿ Chemical properties : Thionic (presence of sulphates or sulphides),
Salic or Sodic, Gypsic or Gypsiric, Calcic or Calcaric, Manganic,
Ferric, Eutric (base saturation > 50%) or Mesotrophic (base satu-
ration > 75%), Humic, Gleyic or Stagnic (with redox features),
￿ Physical properties : Duric, Grumic (well-structured surface layer),
Mazic (massive surface layer),
￿ Colour : Pellic (dark), Chromic (reddish when they are transition-
ing to Nitisols), Albic.
But these qualifiers are either prefixes (important) or suffixes (less
important).
This system treats the Vertisols ( Order ) as most of the other soils, that
is, it introduces the notion of hydric regime at the level of Suborders .
As Vertisols are identified by the suffix 'erts', it distinguishes Aqu-erts,
Ud-erts, Ust-erts, Xer-erts and Torr-erts. Within them, in dry climate,
Great Groups are separated in accordance with the accumulations in the
soil profiles. This corresponds to the presence of Salic, Natric, Gypsic,
Calcic and Petrocalcic horizons. In humid environments, the Great
Groups are characterized by the pH value, electrical conductivity and
signs of hydromorphy ( redox features ).
But at the Suborder level, there are also Cry-erts defined by their
thermic regime, and constituting an exception to the classification of
Vertisols according to hydric regime. In the Cry-erts, there are two Great
Groups, based on content of organic matter.
Soil Taxonomy
6.6 EVOLUTION/DEGRADATION OF VERTISOLS
6.6.1 The Kosselili Sequence
The very beautiful Kosselili sequence is found in Chad in a region where
the annual rainfall is 850-1000 mm. The soil cover contains kaolinite on
the topslopes and, on the footslopes, what has been considered for the
past 30 years or more as a mixture of kaolinite and montmorillonite
(Bocquier 1971). The 2/1 clay mineral is actually a ferruginous smectite.
The substratum is leucogranite (Fig. 6.11).
On the footslope, kaolinite constitutes almost one-half of the upper
part of the soils.
Here, we shall stick to genetic considerations that do not
involve mastery over typology. Lixisols have been defined earlier
Search WWH ::




Custom Search