Reasons for Differences among Soils in Placement of Phosphorus for Maximum Predicted Uptake

Abstract

AbstractPrevious research on placement of phosphate fertilizer has shown that the fraction of the soil fertilized with P affected P uptake by the crop, and that the fraction fertilized for maximum P uptake varied among soils. Placement effects on P uptake have been successfully simulated with a mechanistic uptake model. Values of parameters in the model describing P supply to the root depend on Pli, the initial concentration of P in the soil solution, and Psi, the initial P concentration of the solid phase that rapidly equilibrates with Pli. The objective of this research was to determine the importance of differences among soils in changes in Pli and Psi with increasing rate of P added, on the effect of P placement. Previous research has shown that, as P rate in the fertilized soil increases proportionally with a reduction in the fraction of the soil fertilized, Pli increased curvilinearly and Psi increased linearly. These relations were de‐on calculated P uptake with differing P placements was examined rate of P added, a, c and h are constants and d and g are predicted values of initial Pli and Psi, respectively. The effect of varying the coefficients a and c for the Pli function and of h for the Psi function on calculated P uptake with differing P placements was examined in this study. Standard deviations for c and h of 25% of the mean for 33 diverse soils were obtained from previous research. However, these data also showed that log a was correlated negatively with c (r = −0.92). Hence, in a study of curvilinearity of Pli with P added, it was necessary to adjust log a as c was changed. When log a was decreased as c was increased, the fraction of the soil to fertilize for maximum uptake decreased and predicted P uptake with optimum placement increased. When h was increased while holding c and log a constant, there was only a small increase in predicted P uptake and no change in fraction of the soil to fertilize for maximum P uptake. Hence, changes in Pli had a greater effect on predicted P uptake with changes in P placement than changes in Psi.