DOES THE NITRATE FRACTION ACCOUNT FOR DIFFERENCES BETWEEN DUMAS-N ANDKJELDAHL-N VALUES IN VEGETABLE LEAVES

With recent advances in nitrogen (N) analyzers, the Dumas method may r eplace the traditional Kjeldahl method for the routine diagnosis of N in plants. Because of its nature, the Dumas method truly determines to tal N. The Kjeldahl method only converts protein N and some nitrate (N O3-N) into ammonium. Therefore, the N-NO3- fraction may explain the di fference observed between Kjeldahl-N (Kn) and Dumas-N (Dn) values. Thi s study was conducted to (1) determine the Kn:Dn ratio for vegetable c rops and (2) evaluate the effect of the size of the nitrate fraction o n 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., R-2=0.71,p<0.01). For all vegetable crops combined, th e 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 Kje ldahl 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 alo ne does not account for the difference between Kjeldahl-N and Dumas-N.