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 examine d using paired sites to assess the extent of yield decline with time u nder 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%) wh ere continuously cropped sites were compared with sites which had eith er 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 w ater use under conditions of late-season water deficit to negate, or e ven reverse, early growth advantages on previously untilled soil. At l east part of the observed yield reduction on continuously cropped soil was due to nutrient deficiencies resulting from depletion of both sur face and subsurface reserves during cropping. Long-term cropping has r esulted in depletion of soil K and Zn (especially in the subsoil), org anic carbon and total N status, and caused significant acidification o f both surface and subsoil layers despite the use of lime. The decline in subsoil K status and falling subsoil pH have severe implications f or crop performance in dry seasons, when crops rely on subsoil reserve s to sustain crop growth. The decline in soil N status has occurred de spite a high frequency (>50%) of grain legumes in the crop rotations p ractised on all farms monitored, and illustrates the small N return fr om these crops under rainfed conditions. The reduction in soil organic carbon due to cropping was extreme, with continuously cropped areas h aving organic carbon levels of only 0.9 to 1.5% in the 0-10 cm layer-v alues which were only 25-40% of levels in untilled soil. Grazed grass leys were only partly successful in restoration of soil organic carbon status.