The contibutions of soil properties to cassava yield parameters in Sub-Saharan Africa
The Collaborative Study of Cassava in Africa (COSCA) undertook cassava yield and soil fertility surveys in sub-Saharan Africa in 1991 with the objectives of obtaining average yields from farmers' fields and determining factors which could account for the yield differences across the various climate, altitude, population density and market access zones - the site selection factors considered in the study. In this analysis, the contributions of various soil properties determined from the fields were also obtained and evaluated. The cassava yield parameters considered were fresh root and shoot weight, harvest index and cassava stand density. The interaction between the site selection factors had a greater significant effect on the yield parameters than the individual factors. Whereas the effect of climate was not significant (P = 0.1), both climate x market access and population x market access interactions were highly significant (P 0.01) on root yield. The overall mean root yield was estimated as 13.1 t/ha. It was highest in the subhumid followed by nonhumid and lowland humid zones, and least in the highland humid zone. It was significantly (P 0.05) higher (13.6 t/ha) in the low altitude zones than in the mid (8.5 t/ha) altitude zones. The results of stepwise regression analysis showed that the total contribution of soil variables to root yield variations ranged from about 30% for the entire sub-region through 32% in the lowland and about 40% in nonhumid zones to more than 45% in the highland humid zones. The contribution of total sulphur to the total variations in root yield was consistently high in both the sub-region and across the climate zones compared to other soil variables evaluated. The overall contributions of the soil variables to shoot yield, harvest index and stand density were approximately 35%, 30% and 50%, respectively. Though the contributions of the soil properties to cassava performance confirmed that other factors are also important in cassava production, the significant roles of individual soil variables both in the sub-region and across the climate zones have been established.
Keywords: cassava, soil properties, regression, subSaharan Africa
> Agro-Science Vol. 6 (1) 2007: pp. 1-16