A small ceramic filter candle used as a soil solution sampler is buried in contact with a 2 g urea supergranule to study the fate of the nitrogen source and its relation with the rice plant. The time course of ammonia concentration presents two phases : first diffusion lasting six weeks then absorption with the particularity that roots are making use of ammonia at a rate proportional to its concentration. The first order mechanism could be employed to define ability of rice plants for ammonia assimilation. This measurement is then an index of the fertility of the paddy soil.