Deep placement of urea in the form of supergranules is often recommended in transplanted rice cultivation as a means of avoiding nitrogen loss in the environment. As ammoniacal nitrogen produced by urea hydrolysis just after deep application is chemically stable in the anaerobic layer of the paddy soil, it is relevant to study the diffusion-exchange phenomenon affecting this ammoniacal-N. More precisely, numerical calculus was called on to solve the nonlinear diffusion-exchange equation, admitting spherical symmetry in this context. The adsorption isotherm for ammoniacal-N intervening in this equation was a Freundlich isotherm. Experiments performed in the laboratory (untransplanted situation) and in the field (transplanted situation) enabled the monitoring of the time course of ammoniacal-N concentration in the central position. The tortuosity factor was evaluated at 0.80 in the paddy soil. In the field, the diffusion-exchange phase was clearly separated from the ammonia assimilation phase. The limit between the two was evaluated in six replicates. The first-order mechanism of ammonia disappearance is interpreted as the indication of a tight physiological regulation at the carbon-nitrogen interface acting within roots pushed to their limit in growth and assimilative capacity. This limit is here reported at 12 mM ammonia, attained 39 d after point application. This delay is enhanced when the tortuosity factor is reduced. Similar modelling in the context of cylindrical symmetry can be an option to study new urea fertilizers in the form of urea rods.