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The Effect of Associated Salts on Transformations of Monocalcium Phosphate Monohydrate at the Site of Application¹

ABSTRACT

The effect of associated nonphosphatic salts on the transformation of reagent-grade monocalcium phosphate monohydrate (MCP) during dissolution at the granule site was examined in Hartsells fine sandy loam (pH 5.2) and in calcareous Webster silty clay loam (pH 8.3).

The fraction of the added phosphorous that remained as a residue at the granule site varied from 92% when MCP was mixed with CaCO₃, to only 2% when ammonium sulfate was mixed with MCP. Uptake of phosphorus by plants grown on the soil-fertilizer systems after removal of the residues indicated that plant response was influenced by both the quantity of phosphorus actually transported into the soil, and the composition of the solution in which the phosphorus was transported.

The data further support the concept that during dissolution of the granule, the soil has relatively little influence on the chemistry of the reactions taking place in the granule. Consequently, laboratory phase-rule studies may be quite useful in estimating: (a) the fraction of phosphorus that will remain as a residue at the placement site, and (b) the composition of the solution leaving the granule. The latter is of utmost importance in studies of soil-fertilizer phosphorus reactions.

¹ Contribution from the Soils and Fertilizer Research Branch, Division of Agricultural Relations, and the Fundamental Research Branch, Division of Chemical Development, Tennessee Valley Authority, Wilson Dam, Ala.

² Soil Chemist, Research Chemist, and Analytical Chemist, respectively.

Received for publication February 20, 1960. Accepted for publication June 1, 1960.