Relationship between the isotopically exchangeable and resin-extractable phosphate of deficient to heavily fertilized soil

The rate and extent of soil-solution transfer of orthophosphate ions (P-i) depend both on soil solution P-i concentration and on time. This is so both in experiments on sorption-desorption and in those on isotopic exchange. Because the two methods are based on different principles, it is questionable whether they give a similar quantification of P-i transfer. The amount of isotopically exchangeable P-i, E, was determined over periods of 100 minutes and described as a function of both time and P-i concentration in solution for three soil samples taken from field plots having different P fertilizer histories over 26 years of annual application. In separate experiments, amounts of P-i, Q(d), were extracted from soil suspensions using three levels of anion exchange resin strips for periods ranging from 10 minutes to 7 days. Both initial and final solution P-i concentrations were measured. These concentrations and periods of resin-contact were used to predict the difference in E, DeltaE, between the initial and final states of the suspension-resin system using extrapolations of the equations fitted to the exchange data. Under conditions in which the solution concentration of P-i decreased during extraction the DeltaE values and the resin-extracted P-i values, Q(d), were equal. It is thus possible, using the description of E, to predict the amount of P-i released from soil suspension knowing the initial and final solution P-i concentrations and the time it took for the system to pass from the initial to the final state. For the soil studied, identical amounts of mobile P-i, i.e. the P-i which participates in the soil-solution dynamics, may therefore be assessed by either isotopic exchange or desorption.