SUMMARYUrea application to soil raises the pH and ammonium concentration, thus providing ideal conditions for ammonia volatilization. A mechanistic model is presented, which combines the process of ammonia volatilization with the simultaneous transformation and movement of urea and its products in soil, for predicting the concentration profiles of urea, ammoniacal‐nitrogen and soil pH, and ammonia losses, following application of urea.The model consists of continuity equations describing the diffusion and reaction of urea, ammoniacal‐nitrogen and soil base; it takes into account the volatilization of ammonia and the concurrent acidification of the soil surface; and considers a variablePCo2profile due to soil respiration and urea hydrolysis. The derivation of the continuity equations and their boundary conditions, calculations of ammonia volatilization, and appropriate methods for numerical solutions are descri