Nature of the Reactions of Monocalcium Phosphate Monohydrate in Soils: II. Dissolution and Precipitation Reactions Involving Iron, Aluminum, Manganese, and Calcium1
- W. L. Lindsay and
- H. F. Stephenson2
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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