Abstract

The soils selected for this study represented major soil types in Malawi. They exhibited a wide range of physical and chemical properties. Their pH values ranged from 4.7 to 6.7 and their contents of clay fraction (<0.002 mm) ranged from 170 to 500 g kg-1, organic matter from 6.7 to 39.3 g kg-1, free Fe2O3 from 18.9 to 44.7 g kg-1, and free Al2O3 from 17.8 to 45.6 g kg-1. The soils varied widely in their SO4 sorption behaviour. Soil pH was negatively and significantly correlated (P<0.05) with the bonding energy (k) of SO4 by the soil, Langmuir sulphate sorption maximum (b) and maximum buffering capacity (MBC). Soil organic matter was positively and significantly related with k, b and MBC at the same level of significance (P<0.05). Free Al2O3 and Fe2O3 were positively and significantly related (P<0.001) with b and MBC. Free Fe2O3 and Al2O3 were positively and significantly correlated with k at P<0.01 and P<0.001 respectively. About 90 and 91% of the total variations in the sulphate sorption maxima were accounted for by free Al2O3 and Fe2O3 respectively.

Key words: Sulphate, sorption, desorption, Langmuir, hysteresis, affinity.