Determination of plant iron, manganese and zinc by wet ashing procedures

The efficacy of HNO 3 −HClO4 and HNO 3 −HClO4‐HF digestion mixtures for wet ashing of selected plant samples of field and greenhouse origin for Fe, Mn and Zn analyses was studied. The ternary digestion mixture was required for the determination of Fe, but not of Mn and Zn, in six unwashed field‐grown samples, including four botanical Standard Reference Materials from the US National Bureau of Standards. The ternary digestion mixture was also required for the analysis of Fe in both washed and unwashed tops of field‐grown flax (Linum usitatissimum L.). However, washing appreciably reduced plant Fe and Mn, presumably by removing soil contamination. In addition, the superiority of the ternary digestion mixture for Fe analysis was much less for the washed samples. The contents of Fe, Mn and Zn in washed tops of greenhouse‐raised flax and soya beans (Glycine max L. Merr.) were similar with both digestion mixtures. It is recommended that in plant nutrition studies involving Fe, the efficacy of HNO 3 −HClO4 digestion procedures be routinely checked by completely dissolving any siliceous residue with HF.