Adding phosphorus (P) fertilizer increases soil P supply to the plant root by increasing both soil solution P (P1) and soil adsorbed P (Ps). The objectives of this research were to determine the influence of time on P1, Ps, the buffer power (b), and the effective diffusion coefficient for P (De) for two soils receiving seven rates of P fertilizer (0 to 655 mg P/kg) and to predict plant P uptake using the Barber and Cushman model (1981) with respect to changes in P1, b, and De over time. Samples of the surface horizons of two soils (Woodson and Harney) low in available P were used in this study. Phosphorus fertilization consistently increased P1 of the Harney soil at all sampling times and of the Woodson soil at 3 weeks and 6 months. After 6 months, P1 reached a plateau at the high P rates for the Woodson soil. A significant reduction in P1 was found between 3 weeks and 6 months at the highest P rates for both soils. Adsorbed P increased consistently with increasing P additions for both soils at all sampling times. Unlike P1, Ps continued to decrease with time at the high P rates for both soils. The relationship between P1 and Ps was curvilinear at 3 weeks and then linear for both soils at later sampling times. Predicted P uptake decreased with time at low P rates, whereas it remained stable or decreased only slightly at high P rates. Sensitivity analysis showed that predicted P uptake was most sensitive to changes in P1 and then to b and De. At higher P1 levels, predicted P uptake was not sensitive to changes in soil P supply parameters. Evaluation of the effectiveness of decreasing the fertilized soil fraction on predicted uptake showed that the maximum predicted uptake generally occurred when fertilizer was applied to 1.7% of the soil volume, which simulated band application. A practical implication from this study is the finding that most of the reduction in residual P availability occurred between 3 weeks and 6 months, regardless of P application rate or method.