Inclusion of nitrate and nitrite in the Kjeldahl nitrogen determination of soils and plant materials using sodium thiosulphate

Sodium thiosulphate was used in a modification of the regular Kjeldahl method for the determination of total N in soil and plant samples containing NO‐ 3‐N and NO‐ 2‐N. Quantitative recoveries of added 15N‐labelled and unlabelled N0‐ 3‐N (1000 μg N), NO‐ 2‐N (500 μg N), and NO‐ 3‐N (500 μg N) + N0‐ 2‐N (250 μg N) were obtained from soils, plant materials and adenine, even in the presence of water (up to 50 ml H?0/sample) when 5 g of Na2S203.5H20 as 25% aqueous solution was added to each sample before digestion. Moreover, this procedure does not require additional pretreatment of samples as does the salicylic acid‐sodium thiosulphate or KMnO4‐Fe modification of the Kjeldahl method, and therefore saves considerable time