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Plant samples for N analysis are digested in sulfuric acid at a temperature between 360 and 410 degree Celsius. The rate of digestion is accelerated by using copper sulfate as a catalyst, and anhydrous sodium sulfate or potassium sulfate to raise the boiling temperature of sulfuric acid. On completion of digestion (normally takes less than 2 hours), the samples are cooled and diluted as concentrated alkali is to be added to sulfuric acid digest for distillation. The distilled ammonia is quantitatively absorbed in boric acid and titrated against standard acid . For P, K and secondary nutrients dry ashing is carried out usually at an ignition temperature of 550 to 600 degree Celsius followed by its extraction in dilute HCl or H2SO4 for determining various elements. Wet digestion employs oxidizing acids like like HNO3-H2SO4-HCLO4 tri-acid mixture or HNO3-HCLO4 di-acid. Phosphorus in an aliquot can be determined using the method based on molybdophosphoric blue color or by vanadomolybdophosphoric yellow color method. Potassium can be determined using flame photometer alternately AAS can be used for better precision. Ca and Mg can be determined in the di-acid digest of plant samples either using AAS or versene titration. S can be determined either by Turbidimetric method or the barium chromate colorimetric method (Palaskar et al. 191). Fe, Zn, Mn and Cu can be determined using AAS in the di-acid digest plant tissues. Dithizone method (Shaw and Dean 1952) for Zn and Cu, orthophenanthroline method for Fe and Na-paraperiodate method or Mn can be used an alternative colorimetric methods. B can be determined by azomethine-H method. Mo can be determined using graphite furnace based AAS (Soltanpour et al (1982). or using ICP-AES, if available.

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I stogly suggesto Laser Induced Breakdown Spectroscopy for elemental detenmination not only in plants, but also in other food products. Please see the atatchements