An acid dichromate digestion procedure for total nitrogen determination in soils

Modified version of a dichromate digestion procedure, earlier found suitable for simultaneous determination of organic carbon (C) and potentially mineralizable nitrogen (N) in soils, was evaluated for total N determination in 44 diverse soil samples from the semi-arid region of India. The soils belonged to Vertisols and Alfisols and had a wide range in total N contents. The method involves digestion of the soil sample with an acidified dichromate solution for 30 minutes in a digestion tube in a block digester preheated to 165 degrees C followed by determination of N in the digest by steam distillation procedure. The efficacy of the dichromate procedure for determining total N differed for the two soil types. Excellent correlations were observed between the values of total N obtained by the dichromate procedure (Dichro-N) and by Kjeldahl method (Kjeldahl-N) for Vertisols (R-2=0.995; n=21) and Alfisols (R-2=0.952; n=23). The regression equations describing the relationships between the two soil types can be used to estimate Kjeldahl-N from Dichro-N. Both Kjeldahl and the proposed acid dichromate methods were found to more precise in determining total N in Vertisols than Alfisols. The proposed procedure is rapid and simple and can be used for routine total N determination especially, in soils such as Vertisols. Based on our results the acid digestion procedure certainly deserves a wider evaluation with a range of soil types.