Comparison of Kjeldahl and microwave wet digestion methods for determination of total nitrogen content in soil and plant samples

Determination of nitrogen content in soil and plant material is performed by the conventional Kjeldahl wet digestion followed by steam distillation and titration of ammonium (NH4+). Ammonium (NH4+) determined in this procedure is the most reduced form of nitrogen but this method cannot detect oxidized forms of nitrogen such as nitrate (NO3-) and nitrite (NO2-) that might be present in the samples. Furthermore, the Kjeldahl method, despite its various modifications, is still time consuming and expensive. In order to overcome the disadvantages of the Kjeldahl method, a simpler wet microwave digestion technique with 6 different mixtures of oxidizing chemicals to convert all the nitrogen in the samples to NO3- were proposed. The NO3- contents of the digests were determined on a UV-Vis. Spectrometer. The same samples were analyzed by the conventional Kjeldahl method too. Comparison of the results indicate that the conversion of organic-N in plant samples to NO3- were either incomplete or N was lost in other oxidized forms such as gaseous nitrogen (N2). In soil samples the nitrogen contents determined by the oxidizing method were up to four times those of the Kjeldahl method suggestive of the severe interference of the dissolved mineral materials in the soil digests. Therefore, it has been concluded that the proposed method and the mixtures of wet digestion mixtures could not yet be used for determination of total nitrogen in soil and plant samples.Key words: Nitrogen, Kjeldahl digestion, Wet microwave digestion