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SUMMARY A model for predicting the concentration profiles of urea, ammonium and soil pH in a soil column following diffusion from a surface application of urea is developed, using independently derived parameters, and tested experimentally. The following processes within the model were studied separately under the same conditions as those in the diffusion run. The rate of urea hydrolysis as a function of substrate concentration and pH in the soil solution, and the sorption of urea and ammonium by the soil from solution. A theory for the propagation of changes of pH in soils was applied to describe the diffusion of soil alkalinity arising from urea hydrolysis. These processes were linked by three diffusion equations—for urea, NH 4 and soil alkalinity, which were solved numerically using finite difference methods. There was good agreement between experimental and predicted concentrations of urea and NH 4 , and soil pH values at the two times tested.
Singh et al. (Sat,) studied this question.