Some influences of initial sward botanical composition on the responsiveness of Irish grassland to liming

Results from 11 field experiments, which were designed to evaluate the effects of lime, urea-N and phosphorus on a variety of old and more recently established pastures in Northern Ireland, between 1978 and 1981, were re-examined to see if the effects of liming on N offtake had been influenced by the initial botanical compositions of the swards. Responses to liming on N offtake were noticeably season-dependent; positive responses occurred during spring but negative responses occurred during summer. The positive responses were shown to have been directly proportional to the initial content of perennial ryegrass (PRG) in swards and, arguably, resulted because of the improved ability of this particular sward component to compete with the microbial biomass for mineral N (both soil and fertiliser derived), following lime application. Circumstantial evidence suggested that the negative responses to liming in summer, on the N-fertilised treatments, had been due to enhanced losses of urea-N by ammonia volatilization.