Geoscience Reference
In-Depth Information
6
Vertisols
The word Vertisol was coined by Lehman of the University of Ghent and
introduced in 1956 at the Sixth International Congress of Soil Science
at Paris (Eswaran
et al.
1999). In 1960, the Americans adopted it to
characterize one of the major categories of their 7th Approximation. The
term is now universally used in all classifications. It comes from the
Latin
vertigo
(turning movement). These soils are affected by movements
that we shall examine below. They are black, clayey and spectacular in
many ways.
They represent 2.5 per cent of the soils of the world or 335 million ha,
according to FAO. Present in nearly all latitudes, they are most abundant
in the dry subtropical zone where dry and humid seasons alternate.
6.1
TYPICAL PROFILE AND DIFFERENTIATION
Dark, often black, Vertisols contain between 30 and 90 per cent particles
of clay size. Mineralogically, layers of 2/1 type are seen, with basal
spacing around 14 Å and having the possibility of expansion. In older
publications these soils were identified as containing
montmorillonites
(aluminous, slightly magnesian clay minerals). This is very partially
true. Modern methods detect in these soils mostly
ferriferous beidellites
and
nontronites
(Vingiani
et al.
2004). They contain iron substituting for
aluminium in the octahedral position, giving beidellite if the replacement
is partial and nontronite if it is complete. All these clay minerals are
grouped in the family of
smectites
. Vertisols also contain kaolinite, illites
and, sometimes, chlorites. The principal fact is undoubtedly the presence
of
interstratified smectite-kaolinite
composed of assemblages of the two
types of layers in varying proportions.
Distinguishing features of Vertisols
