PHOSPHATE SORPTION APPROACH FOR DETERMINING PHOSPHORUS REQUIREMENTS OF WHEAT IN CALCAREOUS SOILS

Five field experiments involving P application rates from 0 to 66kg P ha-1 were conducted on irrigated wheat at Tandojam, Pakistan. The soils belonged to two great soil groups, Torrifluvent and Camborthid. All soils were calcareous. Olsen-P contents ranged from 3.5 to 6.3 mg P kg-1. Phosphate sorption curves were developed for soils from control (no P) plots at each site. Concentrations of P in solution established by fertilization in the field as estimated from the sorption curves ranged from 0.008 to 0.16 mg P L-1. Actual grain yields were converted to relative grain yields and plotted against corresponding concentrations of P in solution. Yield response to P application was obtained in each experiment. Control plot yields ranged from 57 to 89% of maximum yield of respective experiments. Phosphorus requirements of wheat were 0.032 mg L-1 for 95% yield as determined from a composite yield response curve. Predicted quantities of P required to attain 0.032 mg P L-1 ranged from 18 to 29 kg P ha-1. The results of the study suggest that the P sorption approach can be used as a rational basis for making P fertilizer recommendations for various soil-crop combinations.