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force is required to pull the plough. These soils are, therefore, unsuitable
for agriculture with draft animals. Without irrigation, plants, whether
sown or transplanted, must be established at the end of the rainy season,
to ensure an unseasonal crop .
The Vertisol is a fragile soil for intensive agriculture. A ploughing
when it is too wet leads to compaction. Fortunately it has a natural
tendency to reconstitute good structure when wetting and drying phases
(seasons, irrigations) alternate (Sarmah et al. 1996). But if compaction is
too much there is total impermeability. Water can no longer infiltrate and
the soil reserves for going through the dry season are not replenished.
This is what happens in the harde soils of Northern Cameroon (Masse
et al. 1993). At the same time runoff increases and provokes erosion with
soil loss that may reach 60 tonnes per ha annually. The B horizon ends
up getting exposed at the soil surface with its assemblage of minerals
with alkaline reaction, especially lime nodules. The pH is then very
high (>8.0). Suitable response consists of abandoning deep-ploughing
and installing all sorts of apparatus for blocking the runoff and forcing
the water to infiltrate vertically. However, when the compaction is total,
the soils cannot be recovered with current technology. In Africa, they
are abandoned to savanna with thorny shrubs.
Vertisols, because of the swell-shrink phenomenon occurring in
them, develop colossal forces. If we extrapolate to the field what we
measure on a sample, the pressures obtained could theoretically go as
high as 100 tonnes per square metre (Avsar et al . 2009). Fortunately part
of the energy is dissipated in closing the cracks. But enough remains to
move walls, break roots and channels, and cut road surfaces. In USA,
the annual damage represents millions of dollars (Eswaran et al. 1999).
It is the same in Europe where one finds few Vertisols but lots of soils
with 'vertic' properties. The damage chiefly affects low houses without
strong foundations.
CONCLUSION
Black, very sticky, productive, bumpy, cracked…, Vertisols are very
special and are easily recognized, even without one's ever having met
them. Pebble-free and deep, they are attractive to man but very difficult
to work. They are very different from reddish, kaolinite-rich Lixisols.
But the two are still associated in the Sahelian landscapes. Progressive
desilication explains why. One is depleted of silica and the other, not. It
is a matter of time, topographic position, climate and microclimate.
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