RELEASE OF CADMIUM FROM A TRIPLE SUPERPHOSPHATE AND A PHOSPHATE ROCK IN SOIL

Abstract

Although phosphorous fertilizers containing elevated amount of cadmium (Cd) could affect Cd accumulation in soils and crops, the extent of the effect may depend on the amount of Cd added and the rate of Cd release from those fertilizers. This study examined Cd forms of a triple superphosphate (TSP) and a western phosphate rock (PR) by sequential extractions and the rate of Cd release from the fertilizers by incubation in a silt loam and subsequent extraction with diethylenetriaminepentaacetic acid (DTPA). The lability of TSP Cd was high, as evidenced by its high solubility in water (36.0%) and 1 M HCl (57.8%). The lability of PR Cd, in contrast, was very low, with 0.1% soluble in water, 29.9% soluble in 1 M HCl, and 70.1% in the residual form. Although soil DTPA-Cd was substantially higher when the soil was treated with the TSP than with the PR at the same Cd rate, the difference narrowed as more PR Cd became extractable by DTPA with time, which reflected increased lability of PR Cd after incorporation into the soil. However, substantial amounts of the added TSP Cd (50%) and PR Cd (77.3%) remained not extractable by DTPA after 210 days of incubation. High correlation of DTPA-Cd with DTPA-zinc (Zn) in the TSP- or PR-treated soils suggested concurrent release of Zn with Cd from both P fertilizers. This should be beneficial, considering Zn can compete with Cd during bioaccumulation processes.