Soil acidification and associated aluminium toxicity are critical issues in New Zealand, particularly in high and hill country areas. However, there have been few studies that have examined the key drivers of exchangeable Al concentrations in New Zealand soils. The National Soils Database was used to investigate the relationship between soil chemical, physical and environmental variables and KCl-extractable Al (AlKCl) for New Zealand soils. Soil AlKCl concentrations were strongly associated with base saturation, soil pH, cation exchange capacity, total N, total C and soil order. However, the relationships differed among the three depth zones (0–20cm, 20–50cm and 50–120cm). Soil acidity and high cation exchange capacity contributed to high concentrations of AlKCl in the soil, whereas high base saturation and total C had the opposite effect. Total N decreased with increasing AlKCl in the topsoil (0–20cm), which is likely a response to the effects of Al toxicity on biological N fixation by pasture legumes. Across a pHH2O range of 3.8–6.4, AlKCl was measured at concentrations that can be toxic to sensitive plants (>1.0 cmolc kg−1). Brown Soils and Podzols are likely more susceptible to Al toxicity, with the highest mean concentrations of AlKCl measured across all depth zones.

Soil extractable aluminium (Al) concentrations have a strong impact on the establishment, growthand persistence of pasture legumes. A survey of 21 soil profiles in the Ashburton Lakes catchmentwas conducted to determine the key factors driving extractable Al concentrations. The mean Al (0.02 MCaCl2) 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 forextractable Al (but R2 was low), while landform type was not. The highest Al concentrations in the 20 cm depthzone 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.

Soil acidity and associated aluminium (Al) toxicity severely limit the establishment and growth of legumes in New Zealand high country pastures. A survey of 13 soils differing in location, soil order, parent material and climate, showed soil pH to range from 4.9 to 6.4 and exchangeable Al (0.02M CaCl2) concentrations of

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Soil extractable aluminium (Al) concentrations can have a strong impact on the establishment, growth and persistence of pasture legumes. This has become clear in New Zealand high and hill-country, where legumes are scarce and failing to persist in acid soils with high Al levels. For the last decade a research programme has been conducted at Lincoln University focused on legume growth and persistence in acid, high Al concentration soils. Research has examined several aspects of soil acidity and Al toxicity and screened and evaluated a range of legume species, identifying several that show promise in their growth and persistence under acidic and high Al concentrations, in addition to harsh climatic environments. This paper summarises this extensive body of research and also suggests some future research topics for addressing the growing challenge of increasing soil acidity and soil Al faced by increasing numbers of producers.