SOIL FERTILITY STATUS AND CASSAVA YIELD IN TANZANIA - (I) NUTRIENT LEVELS IN CASSAVA-GROWING SOILS

One of the main objectives of the Collaborative Study of Cassava in Af rica (COSCA) was to obtain data on the cassava production systems in S ub-Saharan Africa. In Tanzania soils grown to cassava and other major arable crops in 45 villages were sampled in order to assess and compar e their fertility status and relate the fertility status to factors wh ich could have given rise to any observed variations. Soils were colle cted from 0-20 and 20-40 cm depth and 16 physicochemical properties we re considered. Both climate and altitude significantly influenced at l east 11 of the soil properties in fields grown to cassava. Human popul ation density and cassava mixture significantly influenced particle-si ze distribution, total nitrogen, organic matter (OM) and exchangeable acidity. The soils of the non-humid and those of the low altitude zone s were respectively, on average, more fertile than those of the subhum id and mid-altitude zones. Comparatively, soils grown to cassava were as fertile as or more fertile than soils grown to other arable crops, except in the cases of Mg++, total exchangeable bases (TEB) and effect ive cation exchange capacity (ECEC) for soils grown to banana or plant ain. Based on the rating scale for optimum cassava performance, only n itrogen was found to be lower than the critical minimum required by ca ssava and thus needs to be applied in most of the soils. On average, t he soils of the entire area sampled were generally medium to high in f ertility.