Nitrification of aqueous ammonia, and its uptake by grass, are slower when the amount of NH3-N injected per unit length of slit is increased and as a result crop yield is spread more evenly over the growing season. Measurements of the amounts of NH4-, NO3- and NO2-N in the soil, made at intervals from February until October, and computed diagrams showing their distribution in soil under grass, are presented. We conclude that, at average current rates of application, most of the aqueous ammonia used for grass in England and Wales is probably not applied along the injection slit in sufficient quantity to allow it to persist throughout the summer.