Nature of the Reactions of Monocalcium Phosphate Monohydrate in Soils: I. The Solution That Reacts with the Soil

The dissolution of monocalcium phosphate monohydrate (MCP) in water was followed over a 17-day period; and the resulting solution was compared with solubility isotherms of the system CaO-P₂O₅-H₂O at 25°C. Shaking excess MCP with water at room temperature resulted in rapid dissolution during the first few minutes. Within an hour and continuing for more than 24 hours, the solution was in metastable equilibrium with newly formed dicalcium phosphate (DCP) dihydrate and undissolved MCP. The composition of the solution later changed as anhydrous DCP slowly precipitated and the first-formed DCP dihydrate dissolved. The final solution was in equilibrium with anhydrous DCP and undissolved MCP. Band placement of MCP in soil was found to initiate a series of reactions. The solution phase from the reaction zone was sampled by means of filter papers separating several 5-mm. layers of Hartsells fine sandy loam. Reduced water vapor pressure at the fertilizer band caused water to move inward from the surrounding soil. This transport of moisture resulted from both liquid and vapor movement. A wetted zone formed in the immediate vicinity of the band. The solution sampled at the fertilizer band approximated very closely the composition of the metastable triple-point solution (MTPS) in which P is 3.98M, Ca 1.44M, and pH 1.48. This highly concentrated solution moved slowly away from the fertilizer band into the partially dried soil, dissolving Fe, Al, Mn, and other constituents from the soil. In this complex chemical matrix many different phosphate compounds of Fe, Al, Ca, and Mn are expected to precipitate as the solution reacts with more soil and pH rises gradually to that of the surrounding soil. Water continued to move into the fertilizer zone even after MCP had dissolved. This caused dilution of the first-formed solution and enabled it to move farther into the surrounding soil.