Some effects of nitrogen fertilizer on winter wheat

Between 1966 and 1969, 44 experiments on winter wheat tested five rates of N applied as spring top dressings of NH4NO3. There was a marked seasonal variation in the response of grain yield to N with a large response in 1969 (mean optimum 97.5 kg/ha N) and a small response in 1968 (mean optimum 49.2 kg/ha N); the mean optimum N rate for all experiments was 73.1 kg/ha N. The response curves for N and grain yield were classified into 4 types; there were 22 sites with positive curvilinear relationships, 7 were indeterminate, 4 with positive linear and 11 with negative linear relationships. At several of the latter sites substantial yield losses occurred when N was applied. The rate of N that would have been recommended using ADAS prediction methods based on previous cropping and manuring at each site was calculated. At 18 sites the rate of N recommended by prediction methods was within 40 kg/ha of the actual optimum obtained by experiment; at 29 sites it was within 50 kg/ha N but at 8 sites it differed by 75 kg/ha or more. The optimum grain yield obtained by experiment was compared with that obtained when the “recommended” rate of N was given; on 21 sites the difference in yield was less than 300 kg/ha but at 7 sites it was over 900 kg/ha. The results show that the ADAS prediction method accurately assessed the N status at almost half the sites overall. At 11 site (25%) yield differences between the optimum and recommended N rate were greater than 600 kg/ha but if the results for 1969 were excluded because of the unusually high response to N, at only 5 sites (15%) were yield differences greater than 600 kg/ha. Samples of soil from 17 sites were analyzed by 8 methods to assess available N; in 1966 several methods showed promise but in 1967 no method was of value, when correlated with N response. Tiller numbers were counted in 1968 and 1969; there was a strong tendency for more tillers to survive at high N rates. The effects of N on grain size were clear and consistent each year, the highest proportion of the large grain was obtained at low rates of N; each increment of N decreased grain size, irrespective of the optimal N rate.