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

Abstract

Summary. 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 important pasture legume in the region, and other acid sensitive species. However, there was little evidence that cropping over a long time increased soil 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 and usually timbered control areas. The return of cations and associated alkali 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 nitrogen fixing capacity of barrel medic. All but 4 sites had pH_Ca <6, where some reduction in nitrogen fixation would be expected to occur. Variation in pH within transects within paddocks was large, which is consistent with the common observation of poor and patchy growth of barrel medic. Exchangeable calcium, important for Medicago–Rhizobium meliloti symbiosis function, decreased with increasing acidity. The potential for manganese toxicity appeared low with aluminium toxicity likely only at some sites with pH_Ca ≤4.5. Total carbon decreased with increasing soil acidity and was less under cropping at any pH_Ca. Although acidification of these soils due to agricultural activity was not apparent from paired sampling of cropped and contiguous uncropped transects, they seem likely to be prone to acidification.