Geoscience Reference
In-Depth Information
In saline soils of Solonchak type, relatively abundant sulphates may
confer a Sulphatic property.
12.6.5 Reclamation of Mangrove Swamps
Soils of mangrove swamps must be unsuitable for any crop in the
waterlogged state, because of excess water, salinity, lack of oxygen,
unfavourable consistence ('chestnut butter') as well as toxicity of
ferrous iron. Indeed, in a soil submerged for several weeks, ferrous iron
in solution can easily exceed 1000 ppm, of course on condition that iron
is present in the sediments. This is an excessive concentration for many
plants. Among them is rice, the toxicity threshold of which is about 300
ppm (Vizier 1990). Depression of growth may therefore occur. Besides,
salinity is often a constraint (Chap. 13); lastly, the low strength of the
soils limits the possibility of mechanization.
The soils of mangrove swamps, after drying, do not become very
favourable for agriculture: extreme acidity and toxicity of free aluminium
(Bloomfield and Coulter 1973). Aluminium is, as we have seen earlier,
relatively soluble in very acid medium. Laboratory experiments
have shown a depressing effect on plants at concentrations of 1 or
2 ppm. These values are greatly surpassed in acid sulphate soils. At
the same time, there is deficiency of phosphate that is retained on
clay minerals and organic compounds or is coprecipitated with Al
(aluminium phosphates).
But man is ingenious and has sought to restore a condition favourable
for cropping. For this, it is necessary to raise the pH and thus eliminate
sulphates. This is done rather easily by flooding the lands in delta
regions or riverain areas where fresh water is available in plenty. But if
freshwater is lacking, it may be useful to wash the soils with seawater
before ending with a rinsing with better quality water! Seawater, rich
in cations, is also preferable to fresh water for avoiding deflocculation
of the clays (Chap. 13). But as the source of sulphur is very large in
the soil, the operation has to be repeated every year for several years,
without necessarily leading to a final solution. Liming will obviously
be useful to raise the pH and limit acidity but, in developing countries,
the investment capability and infrastructure are most often lacking.
The example of the Mekong delta, in Vietnam, is instructive (Husson
et al
. 2000). Water is abundant. The soils, flooded in the rainy season,
are obviously unusable at that time. But in addition, in the dry season,
they are rapidly oxidized and acidified (sulphides Æ sulphates),
becoming unsuitable for any agricultural planning. In such a context,
