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

Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907

*Corresponding author.

ABSTRACT

Previous 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 P_li, the initial concentration of P in the soil solution, and P_si, the initial P concentration of the solid phase that rapidly equilibrates with P_li. The objective of this research was to determine the importance of differences among soils in changes in P_li and P_si 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, P_li increased curvilinearly and P_si 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 P_li and P_si, respectively. The effect of varying the coefficients a and c for the P_li function and of h for the P_si 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 P_li 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 P_li had a greater effect on predicted P uptake with changes in P placement than changes in P_si.

This research was supported in part by a grant from the Fluid Fertilizer Foundation. Contribution of Purdue Univ. Agric. Exp. Stn. Purdue Journal Paper no. 11 847.

Received for publication December 20, 1988.

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