Factors affecting the fertility status of soils growing cassava in sub-Saharan Africa

The soils of cassava-growing areas of sub-Saharan Africa were sampled in 1991 during the second phase of the Collaborative Study of Cassava in Africa (COSCA) in order to assess and document their fertility status. About 500 fields specifically grown to cassava or cassava-based crop mixtures were sampled and the soils analyzed for nineteen physico-chemical soil properties. The soil data reported in this paper were obtained from Cote d'Ivoire, Nigeria, Tanzania, and Uganda. The variations in soil properties as may be attributed to differences in climate, altitude, depth of sampling, and cassava intercropping systems were also analyzed. The soils used for cassava in these four countries were generally of medium to high fertility status. Here low, medium, and high fertility classes refer to soil nutrient levels where response by cassava to the application of such nutrients is definite, may be obtained, and not expected, respectively. The fertility status across the agroecological zones ranges from high to low in the order: nonhumid>highland humid>subhumid>lowland humid. The soils used for cassava in the mid-altitude zones were significantly more fertile than those of the low-altitude zones. Similarly, soils from 0-20 cm depth contained significantly higher nutrients than those from 20-40 cm depth. Generally, soils grown to sole cassava or where cassava was considered a major or minor crop were statistically similar in terms of their nutrient contents. The nutrient ratings obtained from three scales suggest that the cassava-growing soils are not 'marginal' in terms of both nutrient content and availability. Low levels of organic matter and total nitrogen (N) were the major soil constraints identified. Research needs to address these because the low nutrient reserve is essentially due to low organic matter levels in the soils.