Manganese-Phosphate Solubility Relationships in an Acid Soil
1989
Schwab, A. P.
The chemistry of Mn and P is often complicated by apparent undersaturation with respect to mineral phases. The role of Mn phosphates in controlling the solubility of Mn and PO₄ was studied in acid soils which had been fertilized annually for 40 yr with NH₄NO₃ and superphosphate. Soil pH ranged from 4.1 to 6.8 and available P varied greatly. Fresh, field-moist soil samples were equilibrated with 0.01 M CaCl₂ for 25 d and then analyzed for Mn and PO₄ activities. A method was developed to measure redox potential reliably with a Pt electrode to allow calculation of Mn³⁺ activities. In previous studies, activities of PO₄ in these soils were less than the solubilities of strengite (FePO₄) and variscite (AlPO₄), and Mn activities were less than Mn oxide solubilities. The measured activities of Mn³⁺ and PO³⁻₄ were consistent with the presence of MnPO₄·1.5H₂O but were undersaturated with respect to Mn(II) phosphates. Contribution no. 89-391-J from the Kansas Agric. Exp. Stn.
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