Some effects of the distribution of cadium and phosphate in the root zone on the cadmium content of plants

Abstract

Effects of the distribution of cadmium and phosphate in the root zone on the cadmium content of subterranean clover were examined in pot culture. Addition of cadmium, un~formly mixed throughout the soil, increased the cadmium content of plants at all levels of added cadmium. Plant yield was depressed and toxicity symptoms were evident when cadmium equivalent to 5 ppm, or more, was added to the soil, and yield of roots was depressed by the addition of 1 ppm. When the same amounts of cadmium were added only to the surface 2 cm of soil, plant yield was unaffected except at the highest level (40 mg per pot or 100 ppm in the treated layer) when slight cadmium toxicity was evident. The cadmium content of the plant material was increased, but to a smaller extent than when the cadmium was mixed throughout the soil. At the highest levels of cadmium addition (10-40 mg per pot) the plant cadmium content reached a maximum value of 44 ppm compared with 109-362 ppm for the fully mixed treatments. Root yield below the cadmium treated layer was unaffected, and immobilization of cadmium in the roots growing in the high cadmium layer of soil may have led to the reduced transfer of cadmium to the plant tops. Uptake of cadmium was also affected by the distribution of phosphate in the root zone. Addition of phosphate to the surface of phosphorus deficient soils led to the proliferation of roots in the phosphate-treated layer, which resulted in enhanced cadmium uptake from it, especially at low levels of phosphate application. When cadmium was concentrated in the same layer, increased uptake of the added cadmium occurred. This effect was less evident in soil of high phosphorus status. The cadmium content of plants was also affected by the amount of phosphate applied to soil The effects of phosphate addition varied from soil to soil, and both increases and decreases in plant cadmium content were recorded following the application of phosphate. On some soils decreases in cadmium content appeared to result partially from effects of phosphorus on plant growth.