TOTAL ELEMENTAL ANALYSIS DIGESTION METHOD EVALUATION ON SOILS AND CLAYS

The standard digestion method for total elemental analysis of soil mat erial by the Soil Survey Laboratory (USDA Natural Resources Conservati on Service) uses a 2-mL clay suspension with hydrofluoric acid (HF) in a closed Teflon digestion vessel (method HF-SUS). The accuracy and ef ficiency of elemental recovery by method HF-SUS was compared to: (a) m odification of HF-SUS by use of a dried sample (method HF-DRI); (b) mo dification of HF-SUS by use of a dried sample and HF+aqua regia (metho d HF+AR); (c) sample digestion by Li metaborate fusion (method FUS); a nd (d) microwave digestion of samples with HF+aqua regia in Teflon bom bs (method MICRO). Three replications of three standard reference mate rials (SRMs), fine-earth (<2 mm) from 12 soils, and the clay (<2 mu m) from 10 of those soils were analyzed. Method HF+AR shows the most con sistent statistical agreement with the certified SRM values. Analysis of variance (ANOVA) indicates significant effects (alpha=0.05) for met hod of digestion, nonsignificant effects for method times SRM and meth od times clay, but significant effects for method times fine-earth. Co mposition and/or variability of material are significant factors in th e method of digestion. Method HF+AR yields significantly higher experi mental means of Al2O3, Fe2O3, and K2O contents and oxide recovery (sum mation of experimental means for oxides of all reported elements) than all other methods.