Cashew management and its effect on soils and intercrops: the case of sulphur dusting in South Eastern Tanzania

Abstract

This study examines management of cashew groves in South Eastem Tanzania and the effect of adopting sulphur dusting on soils and on cashew and its intercrops. Production of cashew nuts, an important cash crop in South Eastem Tanzania, is constrained by powdery mildew disease caused by Oidium anacardii Noack To get high yields, farmers have to dust the trees with 90 kg of sulphur per hectare After a decade of sulphur use, there is widespread concern about future productivity of the soils because buffering capacity of the soils to withstand prolonged sulphur use is not known. Production of cashew nuts is also low due to planting in marginally suitable areas The other associated problem is the lack of a strategy to address soil acidity arising from sulphur use The main objective of the study is therefore to find approaches that will ensure sustainable production and management of the cashew-intercrop based farming systems in South Eastem Tanzania. To evaluate soil suitability for the cashew nut tree, henceforth called the cashew tree, soil profile features and physico-chemical properties of soils of 16 representative cashew groves on the Makonde plateau and 14 in the Inland plains were studied. Soil properties indicate that on the Makonde plateau soils arc sandy, highly weathered, deep and devoid of mottles, nodules and hardpans. Ferralsols constituted over 60 % of the soil groupings in cashew groves sampled on the plateau Several soil groupings such as Acrisols, Alisols, Phnthosols, Cambisols and Phaeozems were found in the plains where soils are generally clayey and often shallow and less weathered. Through multivariate analysis, the relationship of 19 soil parameters and 20 cashew tree parameters revealed that tree dimensions were larger and had higher yields on deep, strongly weathered soils most common on the Makonde plateaux On shallow, weakly weathered soils found in the Inland plains, trees had smaller dimensions and low yield. The fact that the plant grew favourably on the plateau, regardless of the low chemical fertility, shows that cashew trees are more sensitive to physical than to chemical limitations of the: terrain. Buffering capacity of cashew growing soils was studied by titrating soil samples from different groves with acid and relating changes in pH to soil properties. Buffering capacity was strongly and positively correlated with percent clay and weakly with percent organic carbon of the soils. Buffering capacity of soils on the Makonde plateau was comparable to that of soils in the Inland plains. Due to the low initial pH and low clay content of soils of the Makonde plateau, acidification of these soils is more likely to reach to critical levels. To verify to which extent past sulphur dusting affected the soil of farmers’ cashew groves, the pH of 70 sulphur dusted groves was compared to 70 non-dusted groves. The survey indicated that use of sulphur has lowered the pH of soils on the Makonde plateau, while soils of the Inland plains have not been affected. To predict the effects of sulphur use on annual intercrops of cashew trees, twelve 3-year field experiments in which sulphur was applied on maize, sorghum and cowpeas were conducted at three locations. Sulphur rates varied from 0 to 240 kg ha’1. Results showed that sulphur decreased germination percentage and grain yield of sorghum and maize, beginning from the second year of application of 120 kg ha'1 and above The decrease was most pronounced in soils on the Makonde plateau. Cowpeas were tolerant to sulphur use. Through an incubation experiment set out to evaluate the ability of Mikindani lime (burned coral lime), Minjingu rock phosphate and ash from cashew leaves to neutralise soil acidity, Mikindani lime was found to be the most suitable material. It raised the soil pH from 3.8 to 6.0 at the rate of 0.3 ton ha’1 costing USS 10 for procurement and application in the field. Both Minjingu rock phosphate and ash required large amounts of materials, resulting in a higher cost. This study has shown that the most important soil properties to check when planting cashew trees are soil depth and weathering status. Deliberate effort should be made to reduce quantity of sulphur used on the Makonde plateau, as the risk for adverse effects of acidification is highest here As risk for soil acidification is less pronounced in the Inland plains, sulphur use can be continued, however, periodic monitoring of soil pH is recommended. To lessen the acidifying effect of sulphur, its use can be reduced by applying crop cultural practises, such as pruning and burning of infected twigs and leaves. Organic fungicides can be an alternative to sulphur but they have the disadvantage of being more expensive and more toxic to humans and animals than sulphur. Although burned lime has proven to be effective to raise the pH of the acidified soils, further research is needed to investigate its effect on the yield of cashew trees and cashew intercrops. An integrated approach to address the mildew problem should also involve a long-term strategy to evaluate/breed for disease resistant tree types.