Evaluation of mixed cation–anion exchange resins as extractants for potentially available micronutrients in tropical soils

We examined the possibility of using mixed cation–anion exchange resinspre-treated with two organic acid ligands, acetate (R-ACET) and tartrate(R-TART) for extraction of phytoavailable Fe, Mn, Cu and Zn using theconventional diethylenetriaminepentaacetic acid (DTPA) as a referenceextractant. In each soil, variable amounts of Fe, Mn, Cu and Zn were extractedby the different extractants. The amounts of Fe, Mn, Cu and Zn did not followprediction based on stability constants of metal-organic complexes. The R-ACETwas a slightly better predictor of soybean uptake of Cu and Mn, and dry matter(DM) yield responses to Cu than either DTPA or R-TART, though differencesbetweenR-ACET and DTPA were not significant. However, soybean DM yield response to Znwas best predicted by DTPA, while Zn uptake correlated more strongly with DTPAthan R-ACET and R-TART extractable Zn. These results suggest that R-ACET may beused as an alternative extractant for Fe, Mn, Cu and Zn in strongly weatheredtropical soils, but it was not superior to DTPA.