Prediction of phosphorus requirements of Desmodium intortum cv. greenleaf using a phosphorus sorption index and extractable phosphorus

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Abstract
The rates of applied phosphorus required for 90% maximum yield of Desmodiim intortum cv. Greenleaf were calculated from pot experiments using 24 fertilized and unfertilized soils from the Atherton Tableland, Queensland, Australia. Phosphorus required was highly correlated (r2 = 0.94) with the phosphorus sorbed (P sorbed) by the soils at a supernatant solution P concentration of 0.08 ppm. P sorbed was found to be a function of phosphorus buffer capacity at 0.08 ppm ("PBC") and phosphorus extractable by acid (0.005 M H2S04) or bicarbonate (0.5 M NaHCO3). PBC was highly correlated (r2 = O.84) with a phosphorus sorption index ("PSI") derived from one addition of 500 μg P g‐1 soil. Combining PSI with acid or bicarbonate extractable P in a multiple regression equation allowed the estimation of phosphorus required with multiple correlation coefficients of R2 = 0.80 and R2 = 0.83 respectively.