A field survey of soil pH and extractable aluminium in the Ashburton Lakes Catchment, New Zealand

Soil extractable aluminium (Al) concentrations have a strong impact on the establishment, growth and persistence of pasture legumes. A survey of 21 soil profiles in the Ashburton Lakes catchment was conducted to determine the key factors driving extractable Al concentrations. The mean Al (0.02 M CaCl₂) concentration was 7.8 mg/kg with the highest values in the top 50 cm of the soil profile. However, there was considerable variation among sites. Landform age, rainfall and depth were all important variables for extractable Al (but R² was low), while landform type was not. The highest Al concentrations in the 20 cm depth zone were found at the wettest sites in the catchment where rainfall was ≥ 1266 mm and where pH was lowest. Farmers in this catchment could use this knowledge to determine which areas of their farms are most susceptible to elevated Al concentrations and at what depth. This would assist in determining which areas could be targeted for development and which are unsuitable