Soil degradation in Central North Cameroon: Water-dispersible clay in relation to surface charge in Oxisol A and B horizons

Water dispersible clay (WDC) is a good indicator for the risk of soil erosion by water. In this study, samples from the A and B horizons of Oxisols have been collected along a toposequence developed on basalt in Central North Cameroon. The aim was to determine the WDC of these horizons and to investigate the relation with soil physical and chemical properties. The effects of pH and different electrolytes on surface charge and clay dispersion were also assessed. Data collected included particle size distribution, water dispersible clay, clay dispersion ratio (CDR), clay flocculation index (CFI), pH, organic carbon content, cation exchange capacity (CEC), exchangeable cations, oxalate and dithionite soluble Fe and Al, mineralogical composition of the clay fraction, specific surface area and surface charge (SC). Clay fractions of Oxisol A and B horizons are constituted mainly by kaolinite, associated with gibbsite and Fe oxides. Total clay content of A horizons varies from 210 to 430gkg⁻¹, and in B horizons from 366 to 405gkg⁻¹. It is negatively correlated to the CEC, which is due to relatively high amounts of organic matter in the A horizons, and also to dithionite and oxalate extractable Fe and Al. The WDC ranges from 103 to 212gkg⁻¹ and from 5 to 98gkg⁻¹ in Oxisol A and B horizons, respectively. The WDC correlated positively with CDR, and both were strongly affected by soluble Na⁺. It appears that A horizons of the upper and middle slope are more dispersible than B horizons. Under varying pH (from 9 to 3), clay fraction from A horizon displayed quite lower surface charge than that from B horizon, being in good agreement with the trend observed by WDC. This finding is assigned to the presence of distinct amounts of organic matter in the A horizon, which, due to its negative charge, increases repulsive forces between soil colloids. Complete flocculation is observed at pH 3 and 4, the pH at which the zero point of charge is reached for clay fractions from A and B horizons, respectively. The multivalent ions Ca²⁺, SO₄ ²⁻ and PO₄ ³⁻ have antagonistic effects on the SC of clay fractions: Ca²⁺ increased SC while SO₄ ²⁻ and PO₄ ³⁻ decreased SC, but complete flocculation within 24h is observed when each of these electrolytes is added even at the lowest concentration of 0.01mML⁻¹. Some measures are suggested to avoid the effects of soluble Na⁺ on the dispersion of these soils.