THE INFLUENCE OF TIME ON PHOSPHORUS SUPPLY CHARACTERISTICS OF TWO MOLLISOLS¹

Abstract

Adding phosphorus (P) fertilizer increases soil P supply to the plant root by increasing both soil solution P (P₁) and soil adsorbed P (P_s). The objectives of this research were to determine the influence of time on P₁, P_s, the buffer power (b), and the effective diffusion coefficient for P (D_e) 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 D_e 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, P_s continued to decrease with time at the high P rates for both soils. The relationship between P₁ and P_s 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 D_e. 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.