PHOSPHORUS RELEASE KINETICS AND EXTRACTABLE PHOSPHORUS AFTER LONG-TERM FERTILIZATION

If different P fertilizers have been applied for a long time, it is difficult to measure the P availability. The objectives of this study were to investigate (i) the kinetics of soil P release by electroultrafiltration (EUF), (ii) extractable P using different soil extractants, (iii) plant P uptake, and (iv) the importance of considering both the P release kinetics and extractable P for plant P uptake prediction. This study was conducted with 15 European Alfisol topsoils with different histories of P and lime applications. The soils had received P as basic slag phosphate (BSP), superphosphate (SP), partially acidulated phosphate rock (PAPR), and phosphate rock (PR) for a long period. Plant P availability was measured in a 4-yr pot experiment with different plants. Phosphorus release kinetics analyzed by EUF were better described by the Elovich equation than by other diffusion functions. The Elovich b values, which can be used as an index of release rate, ranged from 53.5 to 140.0 mg P kg(-1) soil min(-1). These b values were more closely correlated to cumulative plant P uptake (r = 0.96) than P extracted with water (r = 0.90), calcium acetate-calcium lactate-acetic acid (CAL) (r = 0.89), Mehlich 3 (r = 0.81), and double lactate (r = 0.79). Only Elovich b values in the PAPR and PR treatments were significantly correlated to cumulative plant P uptake (r = 0.92). The cumulative plant P uptake of the 15 soils was well predicted with the fitted equation: y = 27.8 + (0.40 CAL P) + (0.83b), r = 0.99, where y denotes the plant P uptake and b is the Elovich 6 value (mg P kg(-1) soil min(-1)) and CAL P is the adsorbed P. This indicates that considering the 6 value as a parameter for P release and the amount of CAL-extractable P for determining P availability following different fertilizer applications is a better estimation of plant P uptake than consideration of only one index of P availability.