Geoscience Reference
In-Depth Information
A profile can be transformed
in situ
to another type. For example,
Leprun (1997) averred that horizons with greenish colour (rich
in smectite) could appear in a Ferralsol
subsequent to
formation
of kaolinitic horizons. Without taking sides on this question, this
hypothesis appears plausible:
F2
, after having caught up with
F1
,
could slow down and be left behind
F1
, in another time period.
This only demonstrates the higher stability of the smectite 'stage'
in a local pedoclimatic environment that has become drier.
The proposed model obviously has limits:
Figure 6.12 does not mean it is necessary to pass the Vertisol
type to reach the Lixisol type even though this path is possible
there. The local pedoclimatic conditions can impose the direct
formation of the latter. There is no necessity to confuse concep-
tual cousinhood and actual filiation.
Based on the appearance of different mineralogical species and
on the notion of the transformation front, our diagram gives the
impression that pedogenesis takes place in jumps and starts.
Actually, chemical and mineralogical analyses most often show
a progression in the internal transformation of the horizons
(Soler and Lasaga 2000). The slow evolution of the interstrati-
fied minerals is the best proof of that. The picture is that of a
wave (as at the seashore) of smectitization followed by a wave
of kaolinitization, without a distinct boundary between them.
These successive overlaps and disappearances of the weathering
fronts in the profile can seem mysterious. They mark simply but
simultaneously the pedoclimatic variations seen vertically in the soil
(see Chap. 3, Fig. 3.1) and the slow changes in the general climate.
6.8 UTILIZATION OF VERTISOLS
Clayey and base-rich, Vertisols exhibit good natural fertility. They are
very suitable for cotton cultivation, and have also been called 'Black
cotton soils'. Actually, cotton has a taproot system and does not have its
roots cut by the development of shrinkage cracks. These soils are also
suitable for annual crops with relatively feeble root development: sugar
cane, sorghum, rice, wheat, barley…
Their particle-size distribution renders them difficult to exploit with
limited financial resources. They cannot be ploughed except within
a narrow range of moisture content. Below that they are too hard to
penetrate. Beyond the range they become terribly sticky so that enormous
