Estimation of the loss rate of ammonium nitrogen from soil (nitrification) and the effect nitrapyrin has on this biological process are made possible by the development of a computer-based system simulation. Inputs to the model are: site location, soil pH, ammonium nitrogen and nitrapyrin application rates and dates, and percentage of soil organic matter. Outputs are: day of the year and nitrapyrin and ammonium nitrogen remaining in the soil. Rates of nitrification and disappearance of nitrapyrin are influenced strongly by soil temperature. Therefore, a soil-temperature-prediction function is part of the model, and the predicted temperature is, in large part, the independent variable that runs the simulation model.