THE EFFECT OF COMPACTION ON THE GROWTH OF PIGEONPEA ON CLAY SOILS .1. MECHANISMS OF CROP RESPONSE AND SEASONAL EFFECTS ON A VERTISOL IN A SUBHUMID ENVIRONMENT

Field experiments were conducted on a vertisol at Dalby, southeast Queensland to investigate the effect of soil compaction on the growth and seed yield of pigeonpea (Cajanus cajan). Deep ripping and roller compaction were used to establish three compaction treatments which represented the range of conditions observed in commercial fields. Trials were conducted with and without irrigation over two growing seasons (1984/1985 and 1987/1988). Growth restrictions resulting from compaction were primarily related to reduced water uptake resulting from decreased infiltration and storage of water, and restricted root growth. Seasonal conditions, in particular the distribution of rainfall, exerted a strong influence on plant response. Yield reductions resulting from compaction varied from 100% in a very dry season to 0% in a wetter season. The reduced impact of compaction in the wetter season was associated with reduced levels of soil strength during early root growth and a decreased reliance on stored subsoil water for growth. The effects of seasonal conditions on plant response make it difficult to predict the yield reduction likely to result from compaction and to identify a critical level of compaction when ameliorative measures are required. A modelling approach is required to assist in the prediction of yield reductions resulting from compaction and to identify seasonal conditions under which yield losses are likely to be most severe.