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

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.

Response of four species of chionochloa and two introduced grasses to soil amendment

“…It can affect the solubility of applied phosphate. large lime dressings on acid soils at least temporarily converting such suitable forms as monocalcium phosphate to less readily soluble apatite-like products (Bouldin et al 1960). 'Lowther and Adams (1970) suggested such an effect to account for depression in clover growth in a closely related soil in New Zealand.…”

Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures

“…As water is absorbed, the solution that forms within a superphosphate granule is supersaturated (Williams, 1971a), leading to the precipitation of sparingly soluble di‐calcium phosphate dihydrate. Depending on the calcium activity, competing reactions (Bouldin et al, 1960), and complexation of inorganic ions, 20 to 34% of the TP may be precipitated as di‐calcium phosphate dihydrate at the granule site (Lehr et al, 1959). Once dissolution is initiated, the highly acidic solution (pH 1.5) diffusing from superphosphate granules dissolves soil minerals, creating a concentrated solution of phosphate, calcium, sulfate, iron, aluminum, and other ions (Lindsay et al, 1959; Lindsay and Stephenson, 1959a, 1959b).…”

The Chemistry and Agronomic Effectiveness of Phosphate Fertilizers

“…Dicalcium phosphate, as DCP (CaHP04) and DCP dihydrate (CaHP04 * 2H20), may precipitate in superphosphate granules (Lehr, Brown, and Brown, 1959;Bouldin, Lehr, and Sample, 1960). The Fe and A l impurities in the phosphate rock used to make the water-soluble P fertilizers may also precipitate in the granule, forming complex Fe and Al phosphates, either during manufacture and storage of the fertilizer, or during dissolution in the soil (Lehr et al, 1967;White, 1974;Gilkes and Lim-Nunez, 1980;Gilkes and Mangano, 1983).…”