Aluminium toxicity in New Zealand soils: Preliminary results on the development of diagnostic criteria

White clover ( Trifolium repens ) was grown at 6 pH levels and 3 rates of applied P on 2 soils: a steepland yellow‐brown earth (yBE), and a brown‐granular clay (BGC), in a glasshouse trial. Aluminium (AI) toxicity limited growth at pH(KCI) <4.0‐4.1. Manganese (Mn) toxicity may also have limited plant growth on the BGC. Increased plant growth at higher pH was attributed to the effects of pH on symbiotic N fixation, and breakdown of organic N. Al toxicity was associated with 1M KCIextractable Al concentrations >1.0‐2.0 me./100 g, 0.02M CaCI,‐extractable Al >3.0‐5.0 μg/g soil, and soil solution Al activity >9‐15μM. Critical values for pH(H.O) and Al in soil solution were dependent on soil type and P treatment.