Determination of phosphorus requirement of cowpea using P-sorption isotherm

Abstract

The P requirement of cowpea in Ikom (Lat 60 50'N, 8 ° 15'E), Bende (50 42'N, 7 ° 44'E) and Ihiagwa (5 ° 21'N, 7 ° 15'E) in Southeastern Nigeria were estimated using the P-sorption isotherms. Soil samples were shaken in graded solutions of P (0, 10, 20, 30, 40, and 50 ppm P), and the P sorbed at 0.2 ppm equilibrium solution P concentration and that required to saturate the adsorption complex of the soils were estimated from the sorption isotherm. Cowpea was grown in polyethylene bags and four P rates representing: (1) Control, (2) P required to attain 0.2 ppm equilibrium solution P concentration, (3) P required to saturate the adsorption complex plus 15 kg P/ha (usually recommended P rate for cowpea fertilization in Southeastern Nigerian soils) and (4) P required to saturate the adsorption complex plus 12 kg P/ha (4 kg P/ha less than the usually recommended P rates) were added. The polyethylene bags were arranged in a randomized complete block design with four replications. The P sorbed at 0.2 ppm equilibrium solution concentrations were 103 ug/g P for Ikom, 9 ug/g P for Bende and 19 ug/g P for Ihiagwa. The P required to saturate the adsorption complex of the soils were 30 kg/ha, 4 kg/ha and 9 kg/ha for Ikom, Bende and Ihiagwa soils, respectively. The P fertilizer rates required for cowpea production in the soils were 45 kg/ha, 19 kg/ha and 19 kg/ha for Ikom, Bende and Ihiagwa soils, respectively. The study shows that the capacity to adsorb P in the soils is low in the order Ikom > Ihiagwa > Bende and suggesting that only a small amount of P fertilizer shall be required for optimum cowpea production in these soils. Organic matter and liming as management practices shall be useful for P fertilizer utilization in the soils since they enhance plant available P by depressing phosphorus adsorption. The use of P-isotherm technique for phosphorus fertilizer determination is therefore, recommended for efficient P fertilization practice in Southeastern Nigerian soils.

Keywords: P-sorption isotherm, organic matter, liming, cowpea

International Journal of Agriculture and Rural Development Vol. 5 2004: 77-85