SOIL FERTILITY STATUS AND CASSAVA YIELD IN TANZANIA - (II) CONTRIBUTIONS OF SOIL PROPERTIES TO CASSAVA YIELD VARIATIONS

During the Collaborative Study of Cassava in Africa (COSCA) survey, ca ssava yields and soil samples were obtained from farmers' fields in or der to assess the yield levels and the contributions of soil propertie s to variations in such yield parameters as fresh root and shoot, harv est index and stand density. The average root yield was estimated as 1 0.5 t/ha, hardest index averaged about 0.50 and stand density was abou t 64,000 per hectare. Both the root and stand density had coefficients of variation above 150%. Each of the yield parameters correlated with at least 13 of the 28 soil properties considered in the simple correl ation analysis carried out. The variations in the yield parameters wer e attributed to the differences in such agro-ecological factors as cli mate, altitude and soil properties. The contribution of soil variables to the variations in all the yield parameters was more than 80% in al l cases. The most important variables were N:P ratio for root yield, c lay content for shoot yield, OM content for harvest index and exchange able Mn for stand density. This analysis indicates that in relating ro ot yield to nutrient levels, nutrient ratios should also be considered because the absolute values of each nutrient may not be the only dete rmining factor.