The distribution in the soil of aqueous ammonia injected under grass

In an experiment with grassland, aqueous ammonia containing about 28% N was injected in rows 305 mm apart either in the autumn or in the spring. Two treatments supplied 126 and 502 kg N ha^?1. Soil representing a cross-section of one row was taken 3 days after injecting the ammonia in autumn and 8 or 9 days after injecting in spring. The plots receiving ammonia in the autumn were resampled in spring. The soil was divided into 51 × 51 mm sections and for the samples at the time of injection those sections close to the point of injection were further subdivided. Each section or sub-section was analysed for NH_4?N and NO_3?N. The array of values was used to produce a computer printout from which lines of equal concentration could be drawn by the procedure described in the appendix. With 126 kg N ha^?1 applied, maximum concentrations found were 900–1440 μg N g^?1, and these had decreased to 100 μg g^?1 at 20–30 mm from the area of maximum concentration. Similarly with 502 kg N ha^?1 maximum concentrations were 2360–3340 μg N g^?1 and the concentration had decreased to 100 μg g^?1 at 50 mm from the zone of maximum concentration. Therefore even with the very large application of fertiliser only a small part of the surface rooting zone contained a concentration of ammonia large enough to damage roots or adversely affect root growth. The zone of maximum concentration was always above the point of injection and usually to the side of the centre of the slit indicating that the aqueous ammonia tended to flow up and to one side of the slit, presumably because the soil closed rapidly behind the narrow injection tine. This shows that aqueous ammonia may not necessarily be effectively injected as deeply as the depth of slit might indicate. The soils resampled in the spring showed that much ammonium nitrogen remained, mostly concentrated round the zone of maximum concentration near the point of injection. Some ammonium had nitrified and nitrate was distributed more uniformly than ammonium, some had moved 300 mm deep. Soil containing most ammonia also contained more water than the bulk of soil. This soil was also plastic and structureless and the clay was deflocculated. The extra water may initially have come from the injected aqueous ammonia but was retained because of the changed structure.