Effects of pH and organic acids on orthophosphate solubility in an acidic, montmorillonitic soil.

In the absence of added organic acid, the solubility of residual o-phosphate in the soil decreased as the pH value was increased from 4 to 7. With o-phosphate additions that resulted in low aqueous-solution o-phosphate concentrations, the soil sorbed more o-phosphate at pH 7 than at 5.5. The solubility data appeared consistent with the presence of an Al-phosphate solid whose formation was initiated through the hydrolysis of exchangeable Al³⁺ and subsequent reaction of the hydrolytic product with o-phosphate in aqueous solution. -from Authors