Geoscience Reference
In-Depth Information
Smectites have a CEC greater than 100 meq/100g. But in the field
these values are not reached (Table 6.1). This is because of the presence
of other minerals in the fine fraction: microdivided kaolinite, illite and
quartz (Robert
et al.
1989).
The soil reaction is variable. Vertisols are often alkaline. They contain
large proportions of calcium and magnesium. The Mg/Ca ratio on the
exchange complex can exceed 2.
In the dry season, these soils exhibit wide shrinkage cracks that can
penetrate deeper than one metre (Fig. 6.1). These cracks disappear in the
humid season.
Fig. 6.1
Shrinkage cracks on the surface of a Vertisol in Senegal.
Scale
: see lens cover.
Photo:
author.
Vertisols can be as much as 4-m to 5-m deep. At first sight, their dark
horizons are scarcely differentiated. But they are clearly distinguished
by their structural properties (Fig. 6.2).
The first horizon has variable structure, sometimes massive when
cultivated; this is seen in box 1. It is often very finely divided under
natural conditions and exhibits an 'expanded' appearance.
In the second horizon, vertical cracks (box 2) yield a very coarse
prismatic structure. The structural units break into smaller, laterally
wedge-shaped peds. These are the
sphenoids
of American authors. They
are seen to the right of the lens cover.
