SUMMARYThe physicochemical and microbiological changes occurring in a fine sandy loam soil following the application of ammonium chloride were followed experimentally and with a simulation model. Two levels of ammonium addition were used corresponding to application rates of 37 and 143 kg ha−1. The measured concentration profiles of ammonium nitrate and pH, which developed in soil columns as a result of the diffusion and simultaneous nitrification of the added NH4+, were measured at different incubation times. The measured profiles suggested that nitrification was inhibited at the site of application of the ammonium salt. This inhibition was attributed to an effect of increased osmotic pressure or chloride ion in the soil. A simulation model was developed to account for the inhibition by examining and testing two hypotheses about the response of nitrifiers to a fluctuating osmotic pressure. These were the irreversible inhibition model, which assumed that exposure to high osmotic pressures irreversibly inactivated a portion of the nitrifier population, and the reversible inhibition model, which assumed that the nitrifiers would recover after exposure to high osmotic pressures. The model included terms for the adsorption equilibria of NH4+and soil acidity with the soil solid phase, and the influence of other ions on the rate of diffusion of each diffusing ion. The inputs to the model were based on parameters obtained independently of the diffusion experiments. Good agreement was found between experimental and predicted concentration profiles for both models although the reversible inhibition model gave the better simulation of the dat