|SPORE Bulletin of the CTA No. 31 - February 1991 (CTA Spore, 1991, 16 p.)|
The Saloum, Gambia, Casamance and Senegal estuaries are set in the midst of vast, desolate stretches of land where only a few wizened trees and clumps of dry and yellowing grass survive. These are the tannest It seems impossible to think that these deserted slopes surrounding the swamps were once the rice-growing domain of the Diolas. But years of drought have taken a dreadful toll since this once rich farming civilization managed to cultivate even the most difficult of soils.
In Senegal the tannes have become saltpans with enormous salt deposits," stated Syaka Sadio, who recently submitted a thesis to the University of Wageningen (Netherlands) on the subject of these infertile lands. But he does strike an optimistic note: "My research shows that there are definite possibilities for forestry and pastoralism there."
Drought means that less fresh water flows down the rivers, so that sea water penetrates further upstream. As a result, the river and creek waters have become extremely saline, and with evaporation the water has become, in some cases, two or three times more saline than sea water. Floods bring disaster for the river basins because when the sun evaporates the water, it leaves a thick deposit of salt across the land.
To this is added the salt contained in the ground water, which, since there is little vegetation to protect the soil against evaporation, is drawn up by the sun to the surface of the soil. Because of the lack of fresh water the ground water becomes more saline, and in turn it also leaves a heavy deposit of salt.
A strict salt-free diet
The wind makes matters worse. It carries minute salt particles which fill the cracks in the soil and form little saltdrifts.
Salt in itself is not always harmful, and many plants can stand a high degree of salinity, but the chemical reactions it initiates in the soil can be harmful. If there is too much salt in the topsoil the iron pyrites (sulphide) present oxydizes, the soil becomes acid, and vegetation struggles even to survive.
Syaka Sadio's thesis, "The pedogenesis of the saline acid sulphated veils of the Sine Saloum tannes, and their potential for afforestation", demonstrates that the tannes need not be barren forever. If the salinity levels can be mastered and reduced, cultivation is not impossible.
Hope for the future of the tannes
The Senegal Institute of Agricultural Research (ISRA), after some initial field trials, tried experimental cultivation at Ngan, Fatick and Keur Mactar. Trees (Eucalyptus and Prosopis), cereals (millet and sorghum), and one legume (groundnut) were planted. The scientists conducting the experiments protected the plants from the saline river water by closing the sideways with small concrete dams and building earthen dykes above the edges of the fields to conserve the rainwater.
They got rid of the salt already in the water by closing off the valves and underground drains from the dykes when the land was at its wettest, in the July-August rainy season. The fresh water was trapped in the fields long enough to dissolve the salt, but as soon as the strength of the sun increased the water was released to avoid evaporation and the salt deposit which would have resulted.
In some places where the fields are covered by a thick layer of dry grasses, millet stalks and sorghum remains, the ground water is not drawn up by the sun and remains deep in the soil.
The techniques described are all simple and inexpensive. "We must put more effort into looking for answers. If we can improve our understanding of these soils then we should be able to exploit them agriculturally in both the medium and the long term. This is all the more crucial because the long-range meteorological prognosis does not suggest any return to the pre-drought situation of before 1971." This is the substance of Syaka Sadio's thesis.
The area of the tannes is getting bigger. In the Fatick region at least half the land is now reduced to salt, pebbles and barren black soil. All the estuarine lands are affected: 400,000ha in the Casamance and around Saint-Louis, 230,000ha in Sine Saloum, 140,000ha in Gambia, plus 8000ha in Niayes.
An early 1983 study in Sine Saloum and the more recent ISRA agricultural trials (although not yet conclusive) suggest that the tonnes can be cultivated. If so, there may still be some hope for this impoverished land.