WHEEL-INDUCED SOIL COMPACTION EFFECTS ON RYEGRASS PRODUCTION AND NITROGEN UPTAKE

Soil and crop responses to zero, small and large tyre/soil contact stresses applied by tractors were studied on a newly reseeded ryegrass sward during 1987 and 1988. The compactive efforts, applied during winter or at the time of spring fertilizer application or at first harvest, were sufficient to increase significantly bulk density in the topsoil layer compared with that in a zero traffic control treatment. The largest increases in soil bulk density occurred after spring-time traffic in the first year (1987) of the experiment. In both years, the resulting soil conditions after heavy compaction were relatively unconducive to primary grass growth and first cut yields; the soil was wetter (less well aerated), of greater strength and colder in the early spring, and warmer in the late spring than the soil in the zero and more lightly compacted treatments. During a relatively wet summer (1987) there were significant benefits to second and third cut yields from minimizing compaction; in the relatively dry summer of 1988 there were no compaction effects at second cut. In both years, impaired uptake of nitrogen was related closely to increased amount of traffic and soil density.