Is sulphur acidifying cashew soils of South Eastern Tanzania?

Cashew nut, a major cash crop of Tanzania, is mainly produced in the South Eastern part of the country. Sulphur dust is widely used for controlling powdery mildew disease caused by Oidium anacardii Noack. This can cause acidification of soils and thus, may threaten the productivity of cashew and its intercrops. To assess the extent and magnitude of the current acidification, pH of topsoil (0-20 cm) and subsoil (20-40 cm) of 70 farmers’ cashew groves where sulphur had been applied for up to 12 years, was compared to 70 similar undusted groves. On average topsoil pH of sulphur dusted groves on the Makonde plateau was 0.2 U lower than of undusted groves; subsoil pH was 0.1 U lower. However, in 29% of the groves the difference in topsoil pH between dusted and undusted groves was between 0.5 and 0.9 U. For groves of the inland plains, no effect could be established. Soil pH on the Makonde plateau decreased with increasing number of years of sulphur use while it remained stable in the inland plains. Soils of the Makonde plateau have a high risk for acidifying to pH levels between 4.5 and 4.0 which could affect the cashew nut production and would be detrimental to intercrops such as sorghum, finger millet, and maize. This is due to the soils’ inherently low pH and low buffering capacity which is linked to their high sand content. Lime could be used to mitigate the effect of sulphur and can be obtained locally from fossil coral limestone, not exploited yet. Alternatively, the incidence of powdery mildew could be reduced by crop husbandry techniques or by using organic fungicides; the first alternative has the disadvantage of requiring farmers to understand aspects of the epidemiology, the second of being more costly and more toxic than sulphur and of requiring water for its application which is scarce on the Makonde plateau.