SINGLE SUPERPHOSPHATE DEPRESSES MOLYBDENUM UPTAKE AND LIMITS YIELD RESPONSE TO PHOSPHORUS IN GROUNDNUT (ARACHIS-HYPOGAEA L) GROWN ON AN ACID SANDY SOIL IN NIGER, WEST-AFRICA

The effect of phosphorus (P) fertilization on dry matter production an d nitrogen (N) uptake of groundnut (Arachis hypogaea L.) was studied d uring the growing seasons of 1989, 1990 and 1991 under rainfed conditi ons on an acid sandy soil in Niger, West Africa. Annual application of 16 kg P ha-1 as single superphosphate (SSP) failed to increase the to tal dry matter production significantly in all three years. Fertilizat ion with SSP increased the concentrations of P and sulfur (S) in shoot s from deficiency to sufficiency levels. It decreased the already very low concentrations of molybdenum (Mo), especially in the nodules, and also the N concentration in the shoot dry matter. With SSP applicatio n, total N uptake declined over three years. Foliar application of P a nd soil application of triple superphosphate (TSP) enhanced dry matter production, N and Mo uptake. Although these acid sandy soils are know n to be deficient in P and S, care must be taken in using SSP in groun dnuts as it may induce Mo deficiency, unless supplementary Mo is appli ed.