EFFECT OF LIMING AND ADDING PHOSPHATE ON PREDICTED PHOSPHORUS UPTAKE BY MAIZE ON ACID SOILS OF 3 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-Cushman nutrient uptake model. The reasons for the differences in P availability were evaluated. As the soil pH increased, values of P(li) (P concentration in soil solution) and P(si) (anion resin exchangeable P) changed independently, and the degree of difference varied with soils. Increasing soil pH generally decreased P(li) but increased P(si). Adding P caused a linear increase in P(si) and a curvilinear increase in P(li); the increase was much greater in the Mollisol than in the Ultisol and Oxisol, probably due to a higher initial P(li) in the Mollisol. Predicted P uptake decreased similarly with increased pH for all soils. Predicted P uptake by maize was controlled more by P(li) than P(si).