Fertilizer response - Marginal rate of return - Nutrient balance calculation - Soil properties

R. J. Carsky1 and M. A. Toukourou2
(1)
International Institute of Tropical Agriculture, IITA-Benin, WARDA – Africa Rice Centre, B.P. 08–0932, Cotonou, Bénin
(2)
Institut National des Recherches Agricoles du Bénin, B.P. 01–884, Cotonou, Bénin
Received: 19 December 2003 Accepted: 29 July 2004
Abstract Market opportunities will drive intensification of cassava production and fertilizer will play a role in this. A trial was initiated on 15 farmers fields (replications) in one village territory in Benin on a relatively fertile sedimentary soil site to identify nutrients limiting cassava yield using nutrient omission plots over three cropping years. There was no response to fertilizer in the first year when fresh root yields in the unamended control averaged 19.1 t ha–1. In the second year, the control yield was 16 t ha–1 and there were significant reductions from withholding P (3.5 t ha–1) and K (2 t ha–1) from a complete fertilizer regime. Nutrient balance after 1 and 2 years (cumulative) showed substantial P and K deficits in unamended plots. In the third year, the control yield was 12.9 t ha–1 and effects of withholding K (5.3 t ha–1), P (5.0 t ha–1) and N (3.0 t ha–1) were statistically significant. Soil K was a significant source of variation in yield in the third year. In the third year of annual nutrient additions soil P and K in the top 0.3 m were increased by 37 and 40%, respectively. Based on the cumulative nutrient balance calculation, the annual application needed to compensate nutrient depletion was 13 kg N, 10 kg P, and 60 kg K ha–1. Partial budget analysis based on these amounts of fertilizer suggested that investment was clearly justified in the third year of continuous cropping at current low cassava prices.