THE ROLE OF ISOTOPIC TECHNIQUES ON THE EVALUATION OF THE AGRONOMIC EFFECTIVENESS OF P-FERTILIZERS

Many isotopic techniques can be applied to determine the relative imme diate and residual effectiveness of P fertilizers. Using isotopes as t racers, the percentage of utilization by plants of the P derived from a fertilizer can be determined. However this is only possible during t he three or four months after the application. Therefore, the P fertil izers may be classified only according to their relative immediate eff ectiveness. To also evaluate residual effect, which can be observed wh en more P is applied than is removed with harvest, isotopes of phospho rus can be used. This residual effect is determined by comparing pool sizes of bioavailable soil P in soils with and without P fertilizer ag ed in soil. The bioavailable soil P pool may be analyzed according to three isotopic experimental procedures which give access to either the A value, or the E value or the L value. The aims, the similarities an d the differences between these three procedures, are examined. Some o f the theoretical and practical constraints of each method are describ ed in this paper; they must be followed in order to obtain reliable in formation for agronomic purposes. A method involves measuring the rate of isotopic exchange of phosphate ions in soil-solution systems maint ained in steady-state. It is now possible to predict the effectiveness of P fertilizers, whatever their chemical form when this method is ap plied on soil samples where P fertilizers were applied.