Variation in the acidity of cropped and uncropped light-textured red soilsof central western New South Wales

The light-textured, red, gradational and duplex soils in semi-arid central western New South Wales were found to be on a pH threshold for suitability for the traditional cultivars of barrel medic (Medicago truncatula), an imp ortant pasture legume in the region, and other acid sensitive species. Howe ver, there was little evidence that cropping over a long time increased soi l acidity. Over the 38 sampling sites. mean pH(Ca) in the soil surface 0-20 cm was significantly higher in cropped compared to contiguous uncropped an d usually timbered control areas. The return of cations and associated alka li in standing timber to the soil surface when trees were pushed and burnt is a plausible explanation. In the surface (0-20 cm) layers of 10 of 38 cropped paddocks, mean pH in 0. 01 mol CaCl2/L was <5, sufficient to severely obstruct growth and the nitro gen fixing capacity of barrel medic. All but 4 sites had pH(Ca) <6, where s ome reduction in nitrogen fixation would be expected to occur. Variation in pH within transects within paddocks was large, which is consistent with th e common observation of poor and patchy growth of barrel medic. Exchangeabl e calcium, important for Medicago-Rhizobium meliloti symbiosis function, de creased with increasing acidity. The potential for manganese toxicity appea red low with aluminium toxicity likely only at some sites with pH(Ca) less than or equal to 4.5. Total carbon decreased with increasing soil acidity a nd was less under cropping at any PHCa. Although acidification of these soi ls due to agricultural activity was not apparent from paired sampling of cr opped and contiguous uncropped transects, they seem likely to be prone to a cidification.