Optimum External and Internal P Requirements of Soybean and Wheat for Maximizing Production on a Phosphorus-Deficient Typic Haplustert

A two-year field experiment was conducted to determine external (soil solution) and internal (plant tissue) P requirement of soybean and wheat crops grown in succession on a Typic Haplustert. Adjusted soil solution P concentrations increased significantly the seed yield of soybean and wheat up to a level of 0.1 mg P VI. Similarly, total P uptake by both the crops increased significantly up to 0.13 mg P VI. Relative yields and relative uptake by the test crops were plotted against solution P concentrations and index leaf P. The computed optimum concentrations of P in solution were 0.10 mg P VI for soybean and 0.11 mg P VI for wheat for obtaining 90 per cent of the yield. Similarly, the optimum external P requirements for 90 per cent of the P uptake were 0.12 mg P L⁻¹ for soybean and 0.12–0.13 mg P L⁻¹ for wheat. The optimum concentration of P was 0.25–0.26 per cent in third fully opened trifoliate leaf of soybean and 0.21–0.25 per cent in flag leaf of wheat.