The effects of four grades of sulfurcoated urea (SCU1, 35.1% nitrogen (N) and dissolution rate in water at 38�C of 10.5%; SCU2, 36.3%N and 25.9% dissolution rate; SCU3, 36.2%N and 1 1.2% dissolution rate; SCU4, 36.8%N and 15.4% dissolution rate) were compared with those of uncoated urea as sources of nitrogen for cereals in nine field experiments in two years. In five experiments at five sites in 1978, and in two experiments at two sites in 1979, comparisons were made between fertilizers topdressed either after sowing (1978) or before sowing (1979). In two further experiments in 1979, comparisons were made between fertilizers banded with the seed or topdressed immediately before or after sowing. Supplementary data on the effect of banding were obtained from a glasshouse experiment. There were no differences between sources in three of the five 1978 experiments. At the other two sites urea was superior to SCU when 50 kg N/ha was applied 2 weeks after sowing. Applications of urea 4 or 6 weeks after sowing gave grain yields, at these sites, up to 69 and 57% higher, respectively, than earlier applications. Apparent recovery of fertilizer nitrogen in one experiment in which it was measured was greater for two SCUs (13.1 and 2l.6%, respectively) than for urea (6+9%), but this was true only for applications at sowing. Urea applied 4 and 6 weeks after sowing resulted in much higher recoveries of fertilizer nitrogen (33.9 and 49.3%, respectively) and was more effective in overcoming leaching losses than was the slow-release SCU. There were no effects of time of application before sowing in the two 1979 experiments, indicating little or no loss of ammonia through volatilization, which precluded a comparison of the effects of the three nitrogen sources used. However, uncoated urea outyielded two SCUs in these experiments, by 7.5 and 6.5% in the first experiment and 5 and 2% in the second, respectively. When uncoated urea was banded with the seed at the equivalent of 70 or 140 kg N/ha all plants in the glasshouse experiment died. SCU at the lower nitrogen rate did not affect wheat emergence or survival but a 30% reduction in plant numbers resulted at the higher rate of SCU2. In one field experiment, uncoated urea reduced plant numbers by 96% compared with 20 and 13% for SCU3 and SCU4, respectively, when applied at 75 kg N/ha. Overall, this study showed no reason to use these grades of SCU in preference to uncoated urea, except where there is a need to band urea-containing fertilizer with the seed.
