Soil acidity and growth of a legume. III. Interaction of lime and phosphate on growth of Medicgo sativa L. in relation to aluminium toxicity and phosphate fixation

Abstract

On some acid sandy loam soils, lucerne required large additions of phosphate to grow normally unless lime was also applied. Plant symptoms, and analyses of plants, soil solutions, and 0.01M calcium chloride extracts, supported the hypothesis that, in the unlimed soils, additions of phosphate overcame aluminium toxicity as well as phosphorus deficiency. Both lime and large additions of phosphate lowered the concentrations of aluminium in the soil solutions and in the plants. Lime did not significantly affect either the equilibrium concentrations or the rates of release of phosphate in the soils. Yet at rates of phosphate adequate in the presence of lime, plants without lime looked phosphate-deficient, grew poorly, and usually contained deficient concentrations of phosphorus. The principal effects were consistent with effects of aluminium and phosphate in solution culture at the same concentrations as those observed in the soils. Aluminium toxicity was associated with 10 to 100 µM concentrations of aluminium in the soil solution and 3 to 10 µg atoms/g dry weight in the plant tops. Phosphorus deficiency was associated with phosphate concentrations of the order of 1µM or less in the soil solutions and 100 µg atoms/g or less in the plant tops.