Manganese Phosphate Equilibrium Relationships in Soils
1986
Boyle, F. W. | Lindsay, W. L.
Solubility diagrams were developed using log K° values for MnHPO₄·3H₂O, Mn₃(PO₄)₂·3H₂O, Mn₅H₂(PO₄)₄·4H₂O and MnPO₄·1.5H₂O. The latter mineral was found to be more stable than hydroxyapatite and thus capable of controlling soil-solution P at pH values <7.2 and pe + pH values between 13.5 and 17. Controlled redox cells were used to maintain suspensions of several soils at fixed redox levels above pe + pH = 13.5. After 28, 56, 84, and 140 d, samples were taken, and the clear filtrates were analyzed. The activities of Mn²⁺ and H₂PO⁻₄ were calculated by speciating total Mn and P in solution. Ionic activity products were calculated and compared to the activity products of pure Mn phosphates. This study helps to explain the beneficial effects of phosphate in alleviating Mn toxicities and the importance of Mn phosphates as reaction products of Mn and P fertilizers in soils.
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