SENSITIVITY TO EQUILIBRATION PERIODS OF PHOSPHATE SORPTION AND ISOTOPIC EXCHANGE METHODS ASSESSING Q I RELATIONSHIPS IN SOILS/

Inorganic reactions supplying soil phosphate (P) to plants depend on t he quantity (Q) of available P and on the solution concentration (inte nsity (I)). A critical factor in the description of Q/I relationships of different soils is the time-dependence of the reactions and the lac k of standardization of methods of observation. Here we compare two me thods for measuring Q/I relationships. Increasing P amounts were added to soil suspensions, and the sensitivity of Q/I to times of equilibra tion from 0.4 to 10 days was tested using: (i) the added P removed fro m solution, i.e., the sorbed P (P-s) vs. the P concentration in soil s olution (C-P); and (ii) the time-dependent amount of isotopically exch angeable P (E) vs. C-P. Relationships were determined for both a tropi cal soil and a temperate soil with low P sorption capacity by adding P to obtain C-P UP to 10 mgL(-1). The C-P, P-s, and E values increased with increasing P rates and decreased with equilibration time. Regress ion constants for P-s vs. C-P curves differ by a factor greater than 2 between 0.4 and 10 days of equilibration, whereas regression constant s for E vs. C-P curves are only slightly (<10%) affected. This is expl ained by the fact that E vs. C-P curves describe the dynamic in the ki netic of gross exchanges between P in solution and both the added P, w hich was sorbed, and the pre-existing P, onto the soil solid phase, wh ereas the P-s v.s. C-P curves only characterize the dynamic in the net exchange of added P after a defined period of equilibration. The use of P-s vs. C-P curves is, therefore, limited to comparisons under iden tical experimental conditions, invalidating many comparisons between p ublished values, whereas E vs. C-P curves allow equilibration time-ind ependent comparisons. Because E vs. C-P curves are not strongly affect ed by times of equilibration, they better assess Q/I relationships of soil P supply, which is necessary to gain insight into the understandi ng and description of phosphate supply to plants.