COMPARISON OF P-AVAILABILITY FROM MONOCALCIUM AND DIAMMONIUM PHOSPHATES USING A MECHANISTIC NUTRIENT-UPTAKE MODEL

Calcium and ammonium phosphates are the most commonly used phosphate fertilizers. Since they differ in some chemical aspects it is important to compare their ability for supplying P to plant roots in different soils. The objective of this research was to compare the predicted effectiveness of monocalcium phosphate (MCP) and diammonium phosphate (DAP) for supply of P to maize in 13 soils. Phosphorus was applied at rates varying with soil from 50 to 400 mg kg-1. Thirty days later P, Ca, pH, and Al were measured in the soil solution and in the solid phase. We calculated buffer power (b) and effective diffusion coefficient (De) for P, and used them, together with solution P (Cli), in the Barber-Cushman mechanistic nutrient model to predict P uptake. Monocalcium phosphate and DAP were similarly effective in supplying P to plant roots. Predicted P uptake differed between fertilizers in only three soils, and maximum differences between fertilizers in Cli or resin-exchangeable P (Csi) in any one soil were always less than 30%. The determinations most highly correlated with predicted P uptake were De (r = 0.93**) and Cli (r = 0.60*). Resin-exchangeable P was not significantly correlated with Cli, De, b or P uptake. Calcium, Al, and pH varied with source of P and soil: soils treated with DAP had lower extractable Al, lower Al in solution, and higher soil pH than soils where MCP was applied. Monocalcium phosphate increased extractable Ca whereas DAP did not affect it. © 1990 Kluwer Academic Publishers.