RATE PROCESSES IN THE DESORPTION OF PHOSPHATE FROM SOILS BY ION‐EXCHANGE RESINS

Summary Phosphate extracted by ion‐exchange resins in the chloride and sodium forms from a deep river‐gravel soil under widely varying conditions is always from the isotopically exchangeable or‘labile’ pool. At any reaction time, a constant fraction of this pool is desorbed by the chloride form of the anion‐exchange resin alone, irrespective of the pH and phosphate manuring of the soil. If, however, a sodium: cation exchange resin is included, increasing fractions of the‘labile pool’ are desorbed with decreasing soil acidity. Phosphate desorption by the anion‐exchange resin alone and with the cation exchange resin is shown to be‘particle‐diffusion’ controlled in the anion exchange resin and neither a‘chemical reaction’ nor a‘film‐diffusion’ mechanism. Over the pH range 4·5–8·5, values between 4·8 and 0·9 × 10 ‐9 cm 2 sec ‐1 were calculated for the interdiffusion coefficient of the phosphate: chloride exchange process in the resin. The isotopically exchangeable phosphate in the soils seems to behave like sparingly soluble or weakly dissociating compounds towards ion‐exchange resins and its rate of desorption depends on the nature and composition of the resins.