External and internal phosphorus requirements of wheat in Rasulpur soil series of Pakistan

A field experiment was conducted in Rabi 2004-2005 on Rasulpur soil series (Typic Camborthid) to determine the P requirement of wheat for obtaining 95% relative yield. Site selection was based on calcareousness and P deficiency. Phosphorus sorption isotherms were constructed to study the behaviour of soil to P application by adding 0, 5, 10, 15, 20, 40, 60 and 80 μg P mL^-1 and were examined by the Freundlich equation. The parameters a (amount adsorbed in μg g ^-1) and b (buffer capacity in mL g ^-1) were estimated by regression of the logarithmic form of the data obtained from the adsorption isotherms. Theoretical doses of P (mg kg^-1 soil) were calculated from this equation to adjust 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40, 0.50 and 1.00 mg P L^-1 with native solution P level as control with and without N and K fertilizer application. Wheat crop was grown according to randomized complete block design with three replications. Maximum wheat grain and straw yield of 3.52 and 3.67 Mg ha^-1 was recorded at solution P level of 0.20 mg P L^-1. Maximum phosphorus concentration in wheat grain and straw was 0.410 and 0.145% which was found at solution P level of 0.50 mg P L^-1, respectively. External solution P requirement was 0.172 mg P L^-1 and internal P requirement found was 0.302% for obtaining 95% relative yield of wheat.