THE ACCUMULATION IN SOIL OF CADMIUM RESIDUES FROM PHOSPHATE FERTILIZERS AND THEIR EFFECT ON THE CADMIUM CONTENT OF PLANTS


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Abstract

At least 80 percent of cadmium impurities in phosphate fertilizers applied during normal cropping practices could be accounted for in the cultivated layers of soils examined with the exception of a very light textured siliceous sand from which about 50 percent of the cadmium had been lost, possibly by leaching. Increases in zinc in these soils varied considerably because of substantial removal of zinc in farm produce and, in some cases, because of accession of zinc from sources other than fertilizers.Fertilizer impurities had increased the original levels of hydrochloric acid-soluble cadmium by more than 10-fold in several soils. In most cases the accumulated residues resulted in considerable increases in the cadmium content of plants grown both in glasshouse experiments and in the field, but radish (Raphanus sativus) grown on two soils which had been limed during the course of cultivation had lower cadmium contents than plants grown on the corresponding original unfertilized soils. This was in spite of substantial accumulation of cadmium in the cultivated soils and was probably due to higher soil pH brought about by liming.In a field experiment uptake by wheat of both native soil cadmium and cadmium residues from superphosphate was substantially increased by the application of ammonium nitrate.In glasshouse experiments uptake of cadmium from soil by plants was shown to be affected by soil pH, by the application of zinc, and by the level of soil phosphorus. Cadmium uptake was highest at low pH and decreased as pH was increased. The level of available phosphorus affected the cadmium content of subterranean clover through an effect on the transfer of cadmium from the roots to the plant tops but had little effect on the total uptake of cadmium.In terms of fertilizer application, however, the effects of phosphate and zinc on the cadmium content of plants would seem to be small compared with those resulting from the input of cadmium supplied as impurities in the fertilizers applied.

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