Soil acidity and associated soil aluminium (Al) toxicity severely restrict the establishment, yield and persistence of legumes in New Zealand high and hill country pastures. This study investigated the relationship between soil chemical, physical and environmental variables and soil extractable Al for New Zealand soils in national, catchment and rhizosphere contexts. The relationship between soil chemical, physical and environment variables and soil extractable AlKCl were investigated using the National Soils Database (NSD). Base saturation (BS), soil pH(H2O), cation exchange capacity (CEC), total nitrogen, total carbon and soil order were strongly associated with AlKCl concentrations and relationships differed among the depth zones (0-20 cm, 20-50 cm and 50-120 cm). In a glasshouse experiment soil extractable AlCaCl2 concentration was strongly associated with lucerne shoot yield. Yield increases were strongly associated lime applications which decreased soil extractable AlCaCl2 concentration to below toxic levels (≤2.5 mg kg-1). In contrast, Caucasian clover (CC) shoot yields were not affected by soil extractable Al concentration, with more consistent yields across the range of pH(H2O) (5.0-7.5) and the P rates applied than lucerne. This study clearly highlighted the potential importance of CC use in the high country, where the growth of more sensitive species such as lucerne is restricted by Al toxicity and identified key variables driving soil extractable Al concentrations in a suite of New Zealand soils, and at different scales. The knowledge generated from this thesis has identified specific sets of conditions (environmental, soil chemical and soil order) that have higher concentrations of extractable Al and therefore areas most likely to be susceptible to Al toxicity in New Zealand. Amy was supervised by Dr Jim Moir and Prof Derrick Moot.
Amy is currently working as a Research Assistant for the Centre for Mined Land Rehabilitation at the University of Queensland, Brisbane, Australia.