SOLUBILIZATION OF PHOSPHATE BY RICE PLANTS GROWING IN REDUCED SOIL - PREDICTION OF THE AMOUNT SOLUBILIZED AND THE RESULTANT INCREASE IN UPTAKE

Previous work has shown that rice plants growing in reduced soil are able to solubilize P by inducing an acidification in the rhizosphere through H+ produced in Fe2+ oxidation by root-released O-2, and by the direct release of H+ from the roots to balance excess intake of cations over anions. In this paper, equations for the diffusion and interaction of P and acid in soil are developed to predict the resultant increase in P uptake by the roots. Good agreement was obtained between the profiles of P and pH in the rhizosphere measured in the previous experiments, and those predicted using the equations with independently measured parameter values. The equations showed that solubilization accounted for over 80% of the P taken up. Measurements of the solubilization parameters in a range of reduced rice soils showed that H+ addition increased the quantity of P that could be desorbed per unit weight of soil and the concentration of P in solution, in all the soils tested. The quantity of P solubilized per unit H+ added at a given solution P concentration varied about 50-fold between soils, with a median of 11.9 mmol P per mol H+. The native soil solution P concentration varied 50-fold (median=0.91 mu M) and the soil pP buffer power (the quantity of P desorbed per unit decrease in -log of the P concentration in solution) varied 100-fold (median = 0.36 mmol kg(-1) pP(-1)); the soil pH buffer power varied 7-fold (median = 0.075 mmol kg(-1) pH(-1)). Calculations indicated that, in most of the soils tested, rice plants would depend upon solubilization for the bulk of their P.