AbstractThe persulphate oxidation (TPN) and the Kjeldahl methods (TKN) were compared for measurement of total N in soils and soil extracts. The TPN method was adapted for optimum recovery and precision in the determination of total N. Measurement of total N with TPN and TKN procedures showed the methods to be comparable for both mineral soils and soil extracts. For soil samples with a high content of C (> 5%) and N (> 0.5%), the total N content was highest with TKN. The Flow Injection Analysis (FIA) was very suitable for anlaysis of nitrate in persulphate digestions. The TPN method combined with FIA estimation of nitrate is recommended for analysis of total N in soils and in soil extracts for microbial biomass determinations.