CAN AN ISOTOPIC METHOD ALLOW FOR THE DETERMINATION OF THE PHOSPHATE-FIXING CAPACITY OF SOILS

This work was undertaken to determine the ability of an isotopic exchange kinetics method to measure the fixation of P by soils. The parameter, PFIX, the ratio of introduced carrier-free P-32 over P-32 remaining in solution after one minute of exchange, was determined on a wide range (n = 127) of samples from surface horizons of temperate and tropical soils. PFIX was compared to two parameters derived from adsorption isotherms, the maximum quantity of adsorbed P (PMAX) and the quantity of P to be adsorbed to reach a concentration in solution of 0.2 mg P/L (P0.2). Results showed that PFIX was at least as good as PMAX and P0.2 to estimate the soil fixing capacity. PFIX was correlated to the clay and iron oxide (Fed) contents, and highly significant correlations with Fed were observed for PFIX than for PMAX or P0.2. The isotopic exchange kinetics method can be recommended to determine the soil phosphate fixing capacity. Moreover, because of its accuracy and the minimal disturbance of the soil-solution equilibrium, the method could be used for routine analysis of soil P-fertility status.