Characterization of soil phosphorus by anion exchange resin adsorption and P³²-equilibration

Summary

Adsorption of phosphate by the anion-exchange resin Dowex-2 was investigated. The resin adsorbed small quantities of P from solution quantitatively. The rate of P-adsorption by resin agitated in solution was proportional to the P-concentration in solution, and was independent of the rate of diffusion of adsorbed P in the resin. When 1 g of soil was shaken continuously with 1 g of resin in 100 ml of water, the rate of P-adsorption by the resin was controlled by the rate of P-release from the soil. Quantities of P adsorbed from soil by resin after different lengths of time were less than those equilibrated with P³² during the same time intervals. The curves showing quantity of P adsorbed vs. time could be satisfactorily described by the hypothesis that there were three simultaneous reactions differing in rate, each reaction being first-order with respect to P. The same was true of the P³²-equilibration data, except that the rate of the slowest reaction was apparently independent of time. In a group of 16 soils, the correlation between P adsorbed by the resin in 2 hours and P-availability to plants in the greenhouse, measured by the isotope-dilution method of Fried and Dean, was 0.95. The corresponding correlation between P extracted by the 0.25N HCl — 0.03N NH₄F extractant of Bray and Kurtz was 0.91.