Soil acidity and growth of a legume. II. Reactions of aluminium and phosphate in solution and effects of alumminium, phosphate, calcium, and pH on Medicago sativa L. and Trifolium subterraneum, in solution culture

Lucerne (M. sativa L.) grew as well in solutions maintained at pH 4 as at pH 5, and its growth at pH 4 was little affected by calcium concentrations above 5 mM when aluminium was not added (< 0. l µM). Phosphate concentrations above l µM were adequate for growth if maintained, and if phosphate was kept below 5 0 µM at pH 4 or below 10 µM at pH 4.5, then aluminium concentrations of the order of 100 µM could be maintained without evident reaction between aluminium and phosphate in solution. Under these conditions, uncomplicated by precipitation or phosphate deficiency in the nutrient solution, aluminium toxicity depressed yields, root elongation, and calcium and phosphate concentrations in shoots and roots, and it made the shoots look phosphate-deficient ; but it could not be remedied by increasing phosphate supply even when this restored plant phosphate to high levels. Only where addition of phosphate caused aluminium to precipitate in the solution did it alleviate the toxicity. Increasing the calcium concentration from 1 mM to 5 mM slightly alleviated the effects of aluminium, except at highly toxic concentrations. T. subterraneum took up more aluminium than lucerne, but tolerated aluminium in the medium better. Adding ethylenediaminetetra-acetate to solutions containing 200 µM aluminium concentrations improved the growth of both species to the extent that concentrations of free aluminium were reduced by chelation.