Approximately 30 crystalline phosphate compounds, in addition to colloidal precipitates of variable composition, were identified as reaction products following the addition of various fertilizer solutions to soils and soil constitutents. The identifications were made by means of X‐ray, petrographic, and chemical analyses. The findings help to clarify the mechanism of phosphate reactions in soils, which includes the dissolution of fertilizer and soil constituents and the precipitation of new products.

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.

Nitrification of N released from cyanuric acid, ammelide, ammeline, or melamine varied inversely with the number of amino groups on the triazine ring. Nitrification of melamine and cyanuric acid powders was slightly greater than solutions and considerably greater than -8 + 12 mesh granules of these materials. Melamine and cyanuric acid solutions perfusing through soil slightly inhibited the rate of nitrate formation from added ammonium, and caused a marked but temporary accumulation of nitrite. More nitrate was

TPS, the solution in equilibrium with monocalcium phosphate monohydrate (MCP) and anhydrous dicalcium phosphate, was reacted with successive increments of Hartsells fine sandy loam (pH 4.6) and Rosebud loam (pH 7.6). The experiments were designed to simulate chemical conditions that may occur in soil surrounding a dissolving granule of MCP. Reaction of soil with TPS (pH 1.01, 4.50M P, and 1.34M Ca) resulted in the dissolution of Fe, Al, Mn, Ca, and other constituents from the soil. As successive soil increments were contacted by the solution, simulating its movement from a fertilizer granule, the dissolution processes continued and pH of the solution increased. The solution soon became supersaturated with respect to certain phosphate compounds that slowly began to precipitate. During these reactions with Hartsells soil, Al in solution reached 0.7M, Fe 0.2M, and Mn 0.009M. With Rosebud soil, the concentrations of Fe and Al were slightly less. TPS in contact with soil dissolved Mn more readily than Al, and Al more readily than Fe. With time and rise in pH, the precipitation of these cations was in the order Fe > Al > Mn. Most of the filtrates obtained from reaction of soil with TPS precipitated solids upon standing. Some of these solids were identified by microscopic and X‐ray analyses as crystalline Fe, Al, or Ca phosphates, while others were colloidal, amorphous precipitates that were not identifiable by these methods.

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