Comparison of Five Digestion Procedures for Recovery of Nutrients and Trace Elements in Plant Tissue

Comparison of five digestion procedures for recovery of nutrients and trace elements in plant tissue were examined. The objectives of this study were to compare five digestion procedures: AOAC (dry ashing), two nitric acid, (NA1, commonly used and NA2, our modification), and two aqua regia (AR1, commonly used and AR2, our modification) procedures for recovery of 15 plant nutrients and trace elements in corn, swiss chard, thorn apple, and barley flour (certified material). Generally, the recovery of various elements depended on the digestion procedure used and the plant species. In most instances, NA2 recovered more phosphorus (P), potassium (K), sulfur (S), and manganese (Mn) than the AOAC or the two AR procedures. Also, overall NA2 recovered more K and magnesium (Mg) than NA1. Extractants AR1 and AR2 recovered more iron (Fe) than the two NA procedures, and the same as the AOAC. The AR2 recovered very similar concentrations of nutrients as the AOAC; and in corn tissue, AR2 recovered more zinc (Zn), chromium (Cr), nickel (Ni), copper (Cu), and boron (B) than the AOAC. The AOAC recovered less S and B than the other procedures tested. For instance, in barley flour, AOAC recovered eight to nine times less S, while in corn tissue AOAC recovered around three times less S than the NA procedures. Overall, there are three important findings: (1) NA procedures may be used for recovery of most elements in plant tissue; (2) AR2 procedure is comparable to the official AOAC method for recovery of nutrients and trace elements in plant tissue, and (3) the official AOAC method used in Canadian plant testing laboratories may underestimate the concentration of S and B in plant tissue.