The effect of superphosphate on the cadmium content of soils and plants

Abstract

The cadmium content of phosphatic fertilizers manufactured in New South Wales ranged from 18 to 91 p.p.m. and was closely correlated with phosphorus content. Superphosphate contained 38-48 p.p.m. cadmium. The evidence suggests that the rock phosphate raw material was the sole source of the fertilizer cadmium. The cadmium in superphosphate was soluble in water and was associated with both the phosphate and sulphate components of the fertilizer. It appeared to be as readily. available to plants as the cadmium in cadmium chloride. Cadmium impurities in superphosphate accumulated in the soil, mostly in the surface 10 cm, principally in a cation exchangeable form, and this increased the supply of available cadmium to plants. This accumulated cadmium was extracted from the soil by hydrochloric acid. Increases of up to five-fold in total soil cadmium were found in soils which had been fertilized with superphosphate, and in one soil there was a 12-fold increase in hydrochloric acid-soluble cadmium. Plant uptake of cadmium from soil was generally small and ranged from 0.4 to 7% of that available. Uptake was greatly influenced by soil type, and addition of calcium carbonate reduced the uptake by subterranean clover from cadmium residues in soil. Plant species differed greatly in their ability to take up cadmium, and the concentration of cadmium varied widely within the plant. Generally only 12-18 % of the cadmium in cereal plant tops was translocated to the grain which had a lower cadmium content than any other part of the plant. The addition of cadmium to soil, either as cadmium chloride or in superphosphate, always increased the cadmium content of the grain of cereals or the edible portion of vegetables. Top-dressing of improved pastures with superphosphate brought about considerable increases in the cadmium content of pasture species, especially that of subterranean clover. Analyses of wheat grain and cereal breakfast foods indicated that, although Australian cereal growing soils have long histories of superphosphate usage, the cadmium content of the grain produced is low, the range found being 0.012 to 0.036 p.p.m. Low values, < 0.010 to 0.015 p.p.m., were also found in processed green peas.