1. Urea containing less than 1% biuret was compared with ammonium nitrate in field experiments on barley, sugar beet, winter wheat and grassland, and with ammonium sulphate on potatoes, in various parts of Great Britain in the years 1957–61.

2. When broadcast on the seed-bed for barley at rates supplying 25–45 lb./acre of nitrogen in twenty-five experiments, urea and ammonium nitrate gave similar mean increases in grain yield on acid soils, while on alkaline soils urea gave smaller increases than ammonium nitrate. The relative effect of the two sources was similar on light and heavy soils and in the east and west of the country.

3. When broadcast on the seed-bed for sugar beet at 50–60 lb./acre of nitrogen in nineteen experiments, mostly on alkaline soils in eastern England, no difference was detected between the mean increases in yield of sugar and tops from urea and from ammonium nitrate.

4. In twenty-two experiments on potatoes urea gave slightly smaller mean increases in yield of tubers than those from ammonium sulphate when applied at 40–60 lb./acre of nitrogen, with a larger difference at 80–120 lb./acre. No influence of soil pH, soil texture, region of the country or method of application (broadcast on the flat or over the open ridges) was detected.

5. When broadcast as spring top dressings in seventeen experiments on winter wheat, mostly in western England and Scotland, at rates supplying 34–45 lb./acre of nitrogen, urea and ammonium nitrate gave similar mean increases in yield of grain.