EFFECT OF pH ON PHOSPHATE ADSORPTION AND ISOTOPIC EXCHANGE IN ACID SOILS AT LOW AND HIGH ADDITIONS OF SOLUBLE PHOSPHATE

Summary Measurements of equilibrium phosphate concentrations, adsorbed phosphate and isotopically exchangeable phosphate (E t ) were made on suspensions of acid soils at initial P concentrations ranging from 1–1000 μM and pH values between 4.2 and 6.8. At low P concentrations (1–100 μM), the affinity of the adsorption reaction was greatest at pH 5.2–5.5. This result, which was inconsistent with the ligand‐exchange hypothesis of Kingston et al. (1967, 1972, 1974), was interpreted in terms of the formation of basic aluminium phosphates of varying composition that depended on the OH/P ratio in solution. At the highest P concentrations (1000 μM), when the affinity of adsorption was least at pH 5.2–5.5, it appeared that basic calcium phosphates were precipitating at pH values > 5.5. In suspensions containing 1–100 μM P initially, values of E t did not change consonantly with values of adsorbed P over the pH range 4.8 to 5.5. It is suggested that the E t values were underestimated due to the presence of labelled complexes in solution which were detected by scintillation counting but not by the analysis for orthophos‐phate. Evidence from solution studies (White et al. , 1976) suggested that as much as one‐third of the phosphate could be complexed with aluminium in solutions between pH 4.7 and 5.4 at P/A1 mole ratios of 0.075 to 7.5. It was apparent that the interpretation of the physical significance of E values in acid soils is complex and requires careful appraisal of the experimental techniques used.