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

摘要

In Cameroon, topomorphic vertisols are widespread in the Sudano-sahelian zone above latitude 9oN. With adepth of about 2 to 2.5 m above the water table, these soils show four main horizons from bottom to top: a dark greyhorizon with hydromorphic patches (B3g), dark grey horizon (B21), dark grey horizon with slickensides (B1) and a surficialgrey 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 mineralsreveals the presence of magmatic and metamorphic minerals from the upper part of the watershed. Quartz grains aremainly angular to sub-angular revealing short fluvial transport distance. The vertisols are thus formed from recent alluvialdeposits from weathering, erosion and deposition of rocks from upper parts of the watershed. Physico-chemically, thestudied soils are characterized by high cation exchange capacity (26-42 meq/100g), high sum of bases (74.30-94.23meq/100g), high base saturation, low organic carbon and a very high C/N ratio. Geochemically, Si and Al are thedominant elements, characterised by a Si/Al ratio range of 2.27 and 2.94. According to this rate, 2/1 clay minerals, namelysmectite, are predominant and their presence confirms the shrink-swell behaviour of the soils. Total base concentrationsare high and could be related to the contrasted climate and the low landscape position. Mechanical analyses revealed veryhigh liquid limits (36-46%), low plastic limits (10-17%), high plasticity indices and high shrinkage limits that permit toclassify those soils as medium to high plasticity inorganic clays. Those soils could be potentially useful in many domainslike the chemical industry, pharmaceutics, petroleum industry, agronomy or environmental protection.