THE POSSIBLE CONSEQUENCES FOR HERBAGE GROWTH OF WATERLOGGING COMPACTED PASTURE SOILS

Soil compaction through intensive stocking with cattle has been found to cause a reduction in herbage production which could be ameliorated by soil aeration. Depression in yield was evident under wet soil conditions suggesting that problems were created by poor aeration. The objectives were to identify and quantify the reduction processes in waterlogged pasture soil and to establish the sensitivity of herbage species to some potentially toxic products. Topsoil passed through a 5 mm mesh sieve and intact soil cores with surface vegetation were incubated anaerobically with or without added mineral-N. Also perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) were grown hydroponically to examine their sensitivity to high concentrations of Mn. Nitrate-N at the rate of 200 mg kg(-1) acted as a redox buffer in soil until it was denitrified. It delayed an increase in exchangeable Mn for around one day and exchangeable Re for at least 3 days. Almost all the water extractable Mn was cationic but most of the Fe was non-cationic. Ethylene reached a maximum concentration of around 5 mu L L(-1) in 3 days and then decreased. Only very small concentrations of acetic acid were produced. Net mineralization of NH4-N occurred during anaerobic incubation at about one eighth of the rate of denitrification and the process was not affected by added NO3-N. Ryegrass was more tolerant than clover of high concentrations of Mn. Depletion of available nitrate through denitrification appeared to be the main reason for a decrease in growth of grass on waterlogged intact cores, but in field conditions where anaerobiosis is probably more severe there may be an additional effect of Mn toxicity to clover although ethylene is unlikely to cause physiological damage.