The effects of times of application and chemical forms on the efficiencies of 15N‐labeiled fertilisers for ryegrass at two contrasting field sites

The effects of time of application and chemical form of nitrogen on nitrogen efficiency were studied at two contrasting field sites. Three forms of fertiliser nitrogen (ammonium sulphate, calcium nitrate and urea) were applied to ryegrass at three different times in spring (February, March and April) on a lowland site (S1) in 1983 and an upland site (S2) in 1984. The first nitrogen application (70 kg N ha−1) was with 15N‐labelled fertiliser. Subsequent applications (50 kg N ha−1 cut−1) were unlabelled. Herbage was harvested during the year on five occasions at S1 and on four occasions at S2. The efficiency of fertiliser nitrogen was evaluated by measuring dry matter yield, % recovery by difference, % utilisation of 15N in the first cut and total % utilisation of 15N in all cuts during one growing season. At S1, recovery by difference and utilisation values for all forms of application were significantly (P<0·01) higher from applications in April than from applications in March or February. Only the total % utilisation data identified any difference between nitrogen forms at S1, ammonium sulphate being significantly (P<0·05) more efficient than calcium nitrate over all times of application. At S2, significant (P<0·1) interactions occurred between times of application and forms of nitrogen for all criteria of nitrogen efficiency. Values for total % utilisation ranged from 10% for calcium nitrate applied in February to 93% for ammonium sulphate applied in April. The Study indicated that the choice of nitrogen form for optimum efficiency wound depend on the prevailing soil and climatic conditions after fertiliser application.