Jc. Fardeau et al., THE ROLE OF ISOTOPIC TECHNIQUES ON THE EVALUATION OF THE AGRONOMIC EFFECTIVENESS OF P-FERTILIZERS, Fertilizer research, 45(2), 1996, pp. 101-109
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.