Nature and Genesis of Vertisols and North Cameroon Management Experience: A Review

Abstract

Vertisols are tropical soils that occur world-wide (335 million ha) with high agricultural and other engineering potentials. However, they are underutilized as most of their physical characteristics make management techniques difficult and highly localized. This paper aims to highlight the characteristics, genesis and management strategies of Vertisols for sustainable use. Two main types of Vertisols can be distinguished: topomorphic Vertisols with A(B)gC or difficult A(B)Cg profile and lithomorphic with A(B)C profile type. Vertisols generally show a dark colour, very high compacity, surficial desiccation cracks when dry, surface ponding when moist and large sub-surface slickensides. They contain at least 30% clay fraction, while smectite, cation exchange capacity and base saturation are very high. Lithomorphic Vertisols are developed on various parent rocks and topographic positions where weathering generates base-rich environments that promote smectite synthesis. The formation of topomorphic Vertisols is favoured by low landscape positions suitable for accumulation of basic cations. The latter cover very extensive surface areas of the globe mainly in tropical, semi-arid to (sub) humid and Mediterranean climates showing contrasting wet and dry seasons, while the former have limited geographical extensions and occur mainly in specific islands and volcanic regions. Swelling and shrinking upon wetting and drying is a major characteristic of Vertisols. Most of their properties and uses are dependent on shrink-swell behaviour. Micromorphologically, plasmic separations occur in all Vertisols especially in the middle part of the profile where shrink/swell is most pronounced. Most Vertisols fall under the A-7-5 and A-7-6 classes typical of inorganic clays of medium to very high plasticity designated as bad clays according to AASHTO. Vertisols present numerous interesting uses in the chemical industry, pharmaceutics, agronomy and environmental protection. Nevertheless, man-made structures on Vertisols are not advisable since large investments are needed to maintain and repair damaged infrastructure. In North Cameroon, vertisols cover about 1 200 000 hectares and agricultural management strategies are mainly tilted at moisture control, but also at fertility restoration.