The differential tolerance of the calcareous and acidic populations of Melilotus alba Medic, to toxic levels of aluminium and manganese was studied since the availability of these micro-nutrients is very much dependent upon the pH of the soil. The reduction in the growth yield at toxic levels of aluminium was more pronounced in the case of the calcareous population and at such high levels the acidic type had a significantly higher yield. Only the calcareous population showed symptoms of toxicity due to excess aluminium and it consisted of chlorotic spots on the leaves, particularly the lower ones. The growth yield of the calcareous population was markedly affected by toxic manganese levels in the nutrient solution; the plants were stunted in growth with highly chlorotic leaves, the leaf blades were cupped and the margins of the leaves were crinkled. On the other hand, though the growth yield of the acidic type was also reduced to a lesser degree, the plants did not show any visual toxicity symptoms. At toxic levels of manganese and aluminium, the uptake of other nutrients such as calcium, potassium and phosphorus was more adversely affected in the case of the calcareous than the acidic types.
It was concluded that the greater tolerance to aluminium and manganese toxicity of the acidic population is partly responsible for its occurrence in acid soils and the absence of the calcareous population from acidic habitats.