SUMMARYThe simultaneous nitrification and diffusion of NH4+, applied as ammonium sulphate to laboratory columns, was followed experimentally and with a simulation model. Ammonium was applied as a fertilizer band at levels equivalent to 69 kg N ha−1to a 1 cm depth. The concentration profiles of NH4+, NO3−, SO2−4and pH were measured in two columns for incubation times of 214 and 286 h. The simulation model provided for the precipitation and ion pair formation of CaSO4, the adsorption equilibria of NH4+and soil acid with the soil solid phase, and nitrifier growth and activity. In general, good agreement was found between the experimental and simulated concentration profiles. The effect of CaSO4precipitation on the diffusion of N was investigated using model simulations of the diffusion of NH4+in the absence of nitrification. The simulations suggested that the reactions of SO2−4in the soil could markedly affect the spread of NH4+from a band of (N