Field experiments comparing ammonium nitrate, ammonium sulphate and urea applied repetitively to grassland

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Abstract
1. Ten experiments were carried out on grassland in various parts of England and Scotland to compare ammonium nitrate, ammonium sulphate, urea with less than 1% biuret and urea with about 4% biuret. The grass was cut a number of times during the season and each nitrogen fertilizer was applied at rates supplying 30 and 60 lb. per acre of nitrogen in spring and after each cut except the last.2. Total yield and yields at most individual cuts of both dry matter and nitrogen in the herbage were increased by each increment of applied nitrogen.3. Ammonium nitrate and ammonium sulphate gave similar dry-matter yields at all cuts in six experiments, while in four on calcareous soils ammonium sulphate gave lower yields than ammonium nitrate at one or more cuts. Ammonium sulphate gave lower yields at the second or third cuts in more experiments than at the first cut.4. Urea with < 1% biuret gave lower dry-matter yields than ammonium nitrate at one or more cuts in eight of the ten experiments. Urea became less efficient relative to ammonium nitrate as the season advanced.5. The efficiency of urea with < 1% biuret relative to ammonium nitrate in each experiment was positively correlated with the ammonia absorption potential of the soil.6. Urea with < 1% biuret and urea with about 4% biuret gave similar yields.

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