A meta-analysis of exchangeable aluminium in New Zealand soils using the National Soils Database

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 (0–20 cm, 20–50 cm and 50–120 cm). 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 (0–20 cm), 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