SUMMARYEquations are given for calculating the initial distribution when a solute is (a) applied at the surface (b) placed below the surface and (c) mixed uniformly in a given depth of top soil. These equations are plugged into a predictive model developed by the authors (Rachhpal‐Singh&Nye, 1986a) to compare the concentration profiles of ammoniacal‐nitrogen and soil pH, and ammonia volatilization losses under the three methods of urea application. Placement of urea gave smaller ammonia losses than uniform mixing in the same depth of soil, which in turn gave smaller losses than surface application. Half‐time for ammonia volatilization was about 6 days irrespective of the method and depth of urea application. Concentration profiles of ammoniacal‐nitrogen and soil pH were more affected by variation in the depth of placement than by depth of mixing. The experimental ammonia volatilization losses and the concentration profiles of ammoniacal‐nitrogen and soil pH agreed very well with those predicted by