Investigating Soil Acidity Associated with Elemental Sulphur Oxidation and Nitrification Process on Soils under Cashew Tree, Southern Tanzania

Abstract

Cashew nut production in Tanzania is threatened by powdery mildew disease (PMD), which is controlled by large application of elemental sulphur (S) dust. There is a concern that the amount of S reaching the soil has acidifying effect, leading to soil fertility degradation. Soil acidification associated with sulphur application on cashew trees was investigated in two major soils groups: arenosols and ferralsols, in Southern Tanzania. Soils investigated were both under histories of long-term organic residue removal, additions of cashew, maize and goat farmyard manure (FYM) residues. Sulphur was added to soils at rates of 14.3 and 28.6 mgSkg-1 soils equivalent to 36 and 72kg S/ha/a representing quantities reaching soils after dusting trees. Soil pH, SO42--S and NO3-N were measured at zero time of incubation, and then after every 7 days for 49 days. Soil pH values, amounts of SO42--S, NO3-N and soil ' s buffer capacities were used to construct proton budgets of soils. After incubations, a slight acidification in untreated soils was associated with nitrification, while increasing application rates of S increased acidification particularly in soils under the history of organic residue removal, maize and cashew residues additions. S oxidation and nitrification process accounted for Net Proton (NP) generated in soils. The contribution of S oxidation to NP ranged from white nitrification process contribution ranged between 28 and 74%. In a Luvic arenosol under the history of goat FYM additions, soil pH depression was small due to high buffer capacity of FYM. The application of potentially alkaline organic residues seems to minimize the risk of soil acidification in these soils, and thus recommended for effective preservation of soil environment.