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 r ecently established pastures in Northern Ireland, between 1978 and 198 1, were re-examined to see if the effects of liming on N offtake had b een influenced by the initial botanical compositions of the swards. Re sponses to liming on N offtake were noticeably season-dependent; posit ive responses occurred during spring but negative responses occurred d uring summer. The positive responses were shown to have been directly proportional to the initial content of perennial ryegrass (PRG) in swa rds and, arguably, resulted because of the improved ability of this pa rticular sward component to compete with the microbial biomass for min eral N (both soil and fertiliser derived), following lime application. Circumstantial evidence suggested that the negative responses to limi ng in summer, on the N-fertilised treatments, had been due to enhanced losses of urea-N by ammonia volatilization.