Geoscience Reference
In-Depth Information
tropics (banana, maize, sorghum, etc.) are adapted to, and thus
tolerant of, acid conditions but are poorly productive. But the
introduction of tolerant cultivars implies investment that many
African farmers cannot do unaided.
5.6.1 Functioning of Natural Forest
The equatorial forest constitutes a considerable biomass: at least 50 t
ha
-1
y
-1
of dry matter (Shanmughavel
et al.
2001), even up to 300 t. Litter
fall represents 5-10 t ha
-1
y
-1
and up to 20 t. Except in particular cases,
it is immediately mineralized. Hundreds of kilograms of each of the
usual mineral nutrients are thus returned to the soil annually.
On the other hand the soil mass is very poor in bases (Ca, Mg,
K, Na), we have already seen why. Also the parent material, often
located at great depth, loses its bases rapidly from the saprolite layer
that immediately overlies it. Under these conditions, it contributes very
little to the nutrition of the forest (Hamdan and Burnham 1996). The
forest thus lives in an almost closed cycle: what is released by the litter
is mostly taken up by the roots (Lelong and Roose 1986), sometimes
totally, under eucalyptus for example (Maquere 2008). But biological
recycling of bases under tropical forest is not sufficient to stop the
depletion and weathering at depth. Drainage water generally contains
substantial quantities of bases (Grimaldi 2004).
When the forest is exploited, the capital of bases of the environment
leaves with the trunks that are removed. The deficit in bases is such that,
in some cases, vegetation takes decades to reestablish. The luxuriant
tropical environment only appears rich.
5.6.2
Cultivation of the Land
One hectare of good equatorial forest burned down by man brings
to the plough layer one tonne each of the minerals nutrients calcium,
magnesium and potassium, up to 200 kg of phosphorus, and several
tonnes of nitrogen (Shanmughavel
et al.
2001). This is enough to
neutralize the acidity, that is, to raise the pH to 5.5 and to reduce the
exchangeable aluminium from 80 per cent to 10 per cent, sometimes even
less. A crop is then possible. But the positive effect of slash-and-burn
does not last more than a few months or years. The residual organic
matter of the forest is mineralized and produces H
+
-ions that reacidify
the soil. After a few cropping seasons, the fertility is reduced to zero. It is
Slash-and-burn agriculture
