2.3 Soils and Vegetation
The zone covers three types of grounds on the Maritime Area
soil chart (ORSTOM, 1967; Institute National des Soils, 1987); there are
ferralitic soils, hydromorphic or not and the vertisoils. Their genesis would
be closely related on topography, climate, and vegetation factors.
2.3.1 Types of soils
- Ferralitic soils, characterized by the
prevalence of the iron oxidation actions and sand argillaceous texture; they
are red and thick soils from 10 to 15 m; they are widest in the zone;
- Hydromorphic soils, located along the river
in a discontinuous way, in the depressions and mouth; muddy clay texture to
sand spreader, they are identified by the red beige color;
- Non-hydromorphic soils, little extended,
they are made of a fine colluvium's from 30 to 70 cm thickness, found in edge
of the plates, and presented in the form of micro-reliefs slightly
corrugated;
- Vertisols, located in the depressions and the
mouth, they are constituted of clays inflating very permeable with water.
According to the agriculture soils study carried out by the
National Institute of the Soils (INS, 1987), these grounds are very permeable
and eroded easily when they are without vegetation.
2.3.2 Vegetation
Study zone shows two vegetable formations, flood zones and
non-flood zones vegetation:
- Flood zones vegetation, it is an
herbaceous vegetable formation with some ligneous which the height does not
exceed 10 m. Theses ligneous are Ceiba pentendra, Uapaca
heudeti, Cola gigantea; the sand banks in the minor bed carry
hygrophilous species like Cyperaceae, tufts of bushes
made up of Mimosa pigra and Phyllantus
reticulatus;
- Non-flood zones vegetation, extending in the
plates and slopes; they are the formations: . Arborescent
higher, 15 m in height populated of Adansonia digitata and
Vitex doniana;
. Arborescent lower and shrubby, composed of
Guinean savannas species with Vitellaria paradoxa and
Terminalia glauxesens;
. Herbaceous, not exceed 2 m height and
dominated by Andropogon gayanus and Panicum
maximum.
These bioclimatic facts justify the savanna field extension
in the sedimentary basin whose complex soil-vegetation is influenced by the
climatic mode and Zio hydrological characteristics.
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