Petrological, Physico-Chemical and Mechanical Characterization of the Topomorphic Vertisols from the Sudano-Sahelian Region of North Cameroon

ABSTRACT

In Cameroon, topomorphic vertisols are widespread in the Sudano-sahelian zone above latitude 9ºN. With a depth of about 2 to 2.5 m above the water table, these soils show four main horizons from bottom to top: a dark grey horizon with hydromorphic patches (B3g), dark grey horizon (B21), dark grey horizon with slickensides (B1) and a surficial grey humiferous horizon (A1) with desiccation cracks. Also, they show a heavy clayey texture, very massive structure, high bulk density, very low porosity and a high compacity. The microfabric of the soils is marked by abundant plasmas, isotic at the surface but birefringent at depth, with numerous stress cutans. The microscopic analysis of heavy minerals reveals the presence of magmatic and metamorphic minerals from the upper part of the watershed. Quartz grains are mainly angular to sub-angular revealing short fluvial transport distance. The vertisols are thus formed from recent alluvial deposits from weathering, erosion and deposition of rocks from upper parts of the watershed. Physico-chemically, the studied soils are characterized by high cation exchange capacity (26-42 meq/100g), high sum of bases (74.30-94.23 meq/100g), high base saturation, low organic carbon and a very high C/N ratio. Geochemically, Si and Al are the dominant elements, characterised by a Si/Al ratio range of 2.27 and 2.94. According to this rate, 2/1 clay minerals, namely smectite, are predominant and their presence confirms the shrink-swell behaviour of the soils. Total base concentrations are high and could be related to the contrasted climate and the low landscape position. Mechanical analyses revealed very high liquid limits (36-46%), low plastic limits (10-17%), high plasticity indices and high shrinkage limits that permit to classify those soils as medium to high plasticity inorganic clays. Those soils could be potentially useful in many domains like the chemical industry, pharmaceutics, petroleum industry, agronomy or environmental protection.