DIGESTION PROCEDURES FOR SIMULTANEOUS AUTOMATED DETERMINATION OF NH4, P, K, CA AND MG IN PLANT MATERIAL

The efficacy of several digestion procedures for mineralizing organic matter from plant tissue was compared, and a rapid and precise method suitable for automated determination of NH4, P, K, Ca and Mg in a single digest was selected. A wet-digestion technique using hydrogen peroxide and perchloric acid as oxidative agents in a sulphuric acid medium was deemed the most advantageous among the tested procedures. Appropriate manifolds were designed to determine the sought plant constituents, present in the digests, by automated instruments. Although, recovered ammonium seemed more accurate and precise with the conventional Kjeldahl procedures, mean recovered ammonium was not significantly higher than the suggested method, when the latter was corrected for oxidized ammonium. Further, significantly larger quantities of K, Ca and Mg were recovered compared to the dry-ashing method, though the differences were small. The suggested procedure was verified with a plant-tissue standard and fortified samples. Further, it has been used to digest and analyze over 7,000 plant tissue samples and a number of manure samples without any problems. Two operators can weigh, digest and analyze 100 samples in an average working day without difficulty. An additional operator can, however, quicken the pace and double the number of daily samples.