EFFECTS OF CONTINUOUS CULTIVATION ON FERROSOLS IN SUBTROPICAL SOUTH-EAST QUEENSLAND .1. SITE CHARACTERIZATION, CROP YIELDS AND SOIL CHEMICAL STATUS

The productivity of Ferrosols used for rainfed agricultural production in the south and central Burnett regions of south-east Queensland was examined in relation to the duration under continuous cultivation. A range of crops grown in on-farm situations during 1986-90 were examined using paired sites to assess the extent of yield decline with time under cropping. The changes in soil chemical characteristics that have occurred during the cropping period were also assessed. All locations showed evidence of a significant reduction in crop growth (50-100%) where continuously cropped sites were compared with sites which had either never been cropped or which had been under grazed grass pasture for >20 years. In the absence of severe late season water deficits, this reduced growth rate was always reflected in lower (21-72%) crop yields at maturity. However, crop dry matter (DM) could interact with crop water use under conditions of late-season water deficit to negate, or even reverse, early growth advantages on previously untilled soil. At least part of the observed yield reduction on continuously cropped soil was due to nutrient deficiencies resulting from depletion of both surface and subsurface reserves during cropping. Long-term cropping has resulted in depletion of soil K and Zn (especially in the subsoil), organic carbon and total N status, and caused significant acidification of both surface and subsoil layers despite the use of lime. The decline in subsoil K status and falling subsoil pH have severe implications for crop performance in dry seasons, when crops rely on subsoil reserves to sustain crop growth. The decline in soil N status has occurred despite a high frequency (>50%) of grain legumes in the crop rotations practised on all farms monitored, and illustrates the small N return from these crops under rainfed conditions. The reduction in soil organic carbon due to cropping was extreme, with continuously cropped areas having organic carbon levels of only 0.9 to 1.5% in the 0-10 cm layer-values which were only 25-40% of levels in untilled soil. Grazed grass leys were only partly successful in restoration of soil organic carbon status.