Previously developed mathematical models based on first and zero order sequential transformations of urea–nitrogen to nitrate by soil urease and microorganisms, based on Michaelis–Menten-type kinetics, were examined in a laboratory soil column with continuous flow. Comparison with data show that the models describe adequately concentration profiles of urea, NH4+, NO2− and NO3−. Apparent rate constants for urea hydrolysis and for NH4+ and NO2− oxidation increased with duration of flow. These increases in rate constants are attributable to microbial growth. A microbial profile developed in the column and was characterized by a decrease in population densities with depth.