Automated instrumental analysis of carbon and nitrogen in plant and soil samples

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Abstract
An automated CHN Analyser was compared with the Walkley-Black and Kjeldahl methods for organic carbon (C) and nitrogen (N). Four organic compounds, twenty nine plant materials and five soils were tested. The CHN Analyzer gave C and N values that were not significantly different (P < 0.05) to the theoretical weight percents of the organic compounds. The Walkley Black method gave soil C values significantly lower (P < 0.05) than those obtained with the CHN Analyzer. The Kjeldahl method gave soil N values significantly lower (P < 0.05) than the CHN Analyzer on three of five soils tested. The discrepancies observed between methods appear to be due to different oxidation efficiencies. CHN Analyzer and Kjeldahl N analyses were not significantly different (P < 0.05) for the plant materials except where samples contained greater than 0.7% NO3-N. Potassium nitrate was also added as a spike to a tall fescue sample. Based on recovery of the spiked NO3-N, the Kjeldahl method was a poor measure of total N for plant materials containing greater than 0.7% NO3-N. The findings suggest the CHN Analyzer can be used for the rapid, accurate and simultaneous determination of C and N in plant and soil samples.