EFFECT OF LIMING AND ADDING PHOSPHATE ON PREDICTED PHOSPHORUS UPTAKE BY MAIZE ON ACID SOILS OF THREE SOIL ORDERS

The effect of soil pH on P availability may vary with soils. We limed acid soils of three soil orders (Mollisol, Ultisol, and Oxisol, all with no significant slope differences) to five pH levels and incubated them for 10 days; then four P rates (0, 175, 350, and 525 mg P/kg) were added to each pH level, and we incubated them again for 21 days before examining the effects of soil pH and P rates on the P-soil reactions and predicted P uptake using the Barber-Cush-man nutrient uptake model. The reasons for the differences in P availability were evaluated. As the soil pH increased, values of PH (P concentration in soil solution) and P8i (anion resin exchangeable P) changed independently, and the degree of difference varied with soils. Increasing soil pH generally decreased Pli but increased Psi. Adding P caused a linear increase in Psi and a curvilinear increase in Pli; the increase was much greater in the Mollisol than in the Ultisol and Oxisol, probably due to a higher initial Pli in the Mollisol. Predicted P uptake decreased similarly with increased pH for all soils. Predicted P uptake by maize was controlled more by Pli than Psi.