A RAPID DICHROMATE PROCEDURE FOR ROUTINE ESTIMATION OF TOTAL NITROGEN IN SOILS

A simple procedure for routine estimation of total nitrogen (N) in soils is described. It involves (a) digestion of the soil sample with an acidified dichromate (K2Cr2O7-H2SO4) solution for 45 minutes in a Pyrex digestion tube in a 40-tube block digester preheated to 170-degrees-C, (b) titrimetric determination of the ammonia-N liberated by direct steam distillation of the digest with alkali (NaOH), and (c) calculation of the total-N value by application of a correction factor (1.1) to the measured NH3-N value. Studies using 14 soils selected to obtain a wide range in total N content and other properties showed that the uncorrected recovery of N by the procedure described ranged from 88.5% to 92.3% and averaged 90.5%. When a correction factor of 1.1 was applied to the results, the recovery of N ranged from 97.4% to 101.5% and averaged 99.6%. The dichromate procedure described is rapid and precise, and it permits at least 80 analyses in a normal working day. The only equipment required for its use is equipment commonly used for total N analysis of soils by the Kjeldahl method (40-tube block digester and steam distillation unit), and the acidified dichromate solution used for digestion of the, soil is the reagent commonly used for determination of organic carbon in soil by a modified Mebius procedure. Besides being simpler and more rapid than the Kjeldahl method, the procedure described has the advantage that it does not require the expensive fume disposal system needed for Kjeldahl digestion.