Effect of soil pH on phosphorus-soil reactions determining phosphorus availability in acid soils

Soil pH has been reported to affect the behavior of phosphorus (P) in soils and phosphate uptake by plants. Research is needed to verify the Barber-Cushman mechanistic nutrient uptake model under varying soil pH's. Conflicting views are held regarding the effect of soil pH on P availability. The effect may vary with soils and the reasons for the differences are not well understood. A pot experiment (Part I) was conducted to evaluate the accuracy of the Barber-Cushman nutrient uptake model for predicting P and K uptake at six levels of soil pH in an acid Chalmers silt loam. This study showed that the Barber-Cushman nutrient uptake model accurately predicted the effect of soil pH on P (Y = 1.7 + 0.97 X, r$\sp2$ = 0.97) and K (Y = 67 + 0.94 X, r$\sp2$ = 0.995) uptake by maize. Three different acid soils varying in their chemical properties were used to evaluate the effect of type of soil with regard to soil pH and P rates on soil supply parameters, P$\sb{\rm li}$, P concentration in soil solution, P$\sb{\rm si}$, P concentration on solid phase, and b, buffer power of soil, and predicted P uptake using the Barber-Cushman nutrient uptake model (Part II). In addition, two different incubation times of pH adjustment, and four different reaction times after P addition were studied to evaluate the effect of aging on pH adjustment and P addition on P$\sb{\rm li}$, P$\sb{\rm si}$, b, and predicted P uptake (Part III). The results of this research show that the Barber-Cushman nutrient uptake model can (i) satisfactorily predict the effect of soil pH on P and K uptake by maize, (ii) show that the reason for the difference among soils in their ability to provide P for absorption by plants as affected by soil pH is mainly due to the effect of soil pH on P$\sb{\rm li}$, and (iii) estimate the effectiveness of added P with reaction time after P addition.