Interferences in the determination of isotopically exchangeable P in soils and a method to minimise them

In soils with a high phosphate buffering capacity or in soils with large amounts of colloidal P in soil solution, E-values often overestimate P availability. These problems have been ascribed to a variety of causes including analytical difficulties in terms of measuring low concentrations of P, colorimetric interferences, and inadvertent measurement of non-isotopically exchangeable colloidal P. We investigated measurement of E-values in 8 soil types, of which 6 had properties likely to give overestimates of size of the available pool of P based on results of previous authors. The potential for overestimation of the E-value due to interferences described above was identified in several soil types. A simple anion exchange resin purification step was introduced to the E-value methodology. The resin adsorbs both 31P and 32P isotopes in proportion to their concentration in the solution and minimises transfer of colloids and Si into the analysed eluant, thereby providing a simple way to increase the eluant P concentration and to avoid analytical interferences.