Does the nitrate fraction account for differences between dumas‐N and Kjeldahl‐N values in vegetable leaves?

With recent advances in nitrogen (N) analyzers, the Dumas method may replace the traditional Kjeldahl method for the routine diagnosis of N in plants. Because of its nature, the Dumas method truly determines total N. The Kjeldahl method only converts protein N and some nitrate (NO3‐N) into ammonium. Therefore, the N‐NO3 ‐ fraction may explain the difference observed between Kjeldahl‐N (Kn) and Dumas‐N (Dn) values. This study was conducted to (1) determine the Kn:Dn ratio for vegetable crops and (2) evaluate the effect of the size of the nitrate fraction on the Kn:Dn ratio. Over the 0.9–7.0% N range, Dn was a good predictor of Kn in vegetable samples. The Kn may be estimated from Dn as Kn=0.68 Dn (n=134 obs., R2=0.71, p<0.01). For all vegetable crops combined, the mean Kn:Dn ratio was 0.75. This ratio suggests that approximately 25% of N in the samples was recovered by the Dumas method but not by Kjeldahl digestion. This percentage is much higher than the actual N‐NO3 foliar content. These results suggest that when N‐NO3 is not known (as in most routine samples), Kn may be estimated from Dn as Kn=0.75 Dn. These results also suggest that under a wide range of NO3‐N, NO3‐N alone does not account for the difference between Kjeldahl‐N and Dumas‐N.