THE INFLUENCE OF TIME ON PHOSPHORUS SUPPLY CHARACTERISTICS OF 2 MOLLISOLS

Adding phosphorus (P) fertilizer increases soil P supply to the plant root by increasing both soil solution P (P-1) and soil adsorbed P (P-s ). The objectives of this research were to determine the influence of time on P-1, P-s, the buffer power (b), and the effective diffusion co efficient for P (D-e) for two soils receiving seven rates of P fertili zer (0 to 655 mg P/kg) and to predict plant P uptake using the Barber and Cushman model (1981) with respect to changes in P-1, b, and D, ove r time. Samples of the surface horizons of two soils (Woodson and Harn ey) low in available P were used in this study. Phosphorus fertilizati on consistently increased P-1 of the Harney soil at all sampling times and of the Woodson soil at 3 weeks and 6 months. After 6 months, P-1 reached a plateau at the high P rates for the Woodson soil. A signific ant reduction in P-1 was found between 3 weeks and 6 months at the hig hest P rates for both soils. Adsorbed P increased consistently with in creasing P additions for both soils at all sampling times. Unlike P-1, P, continued to decrease with time at the high P rates for both soils . The relationship between P-1 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 dec reased only slightly at high P rates. Sensitivity analysis showed that predicted P uptake was most sensitive to changes in P-1 and then to b and D-e. At higher P-1 levels, predicted P uptake was not sensitive t o 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 applicati on. A practical implication from this study is the finding that most o f the reduction in residual P availability occurred between 3 weeks an d 6 months, regardless of P application rate or method.