THE AVAILABILITY TO LETTUCE OF ZINC AND CADMIUM IN A ZINC FERTILIZER¹

Abstract

The availability of cadmium (Cd) in a zinc (Zn) fertilizer is not well understood, particularly if the Zn is added along with the Cd and because Zn can have varying effects on Cd accumulation by plants. This greenhouse study examined how repeat additions at high rates of a granulated Zn fertilizer (G-Zn) containing 469 mg Cd kg⁻¹ and 178 g Zn kg⁻¹ affected Cd accumulation by lettuce (Lactuca sativa L.). A soluble Cd salt, CdCl₂, was included in the study for comparison. After equilibrating a Sultan silt loam (Aquandic Xerochrept) with varying rates of Cd at three lime rates for 3 months, the soil was planted to lettuce. Lettuce yield and Zn and Cd concentrations in the plant and in the soil were determined. The experiment was repeated five times in succession using the same soil. Results from the 2nd, 4th, and 5th experiments are presented and show consistently that lettuce yields were unaffected by the amounts of CdCl₂ or G-Zn added. As with CdCl₂ additions, G-Zn additions increased Cd concentrations in lettuce and the amount of Cd extractable by CaCl₂. Across all experiments, soil total Cd (r² = 0.49, P< 0.001) was less effective than CaCl₂-Cd (r² = 0.70, P< 0.001) in predicting lettuce-Cd unless it was included along with soil pH in a stepwise multiple regression (r² = 0.64, P< 0.001). The added Cd from the G-Zn was not transferred as efficiently to the plant as the Cd from CdCl₂. Repeated additions of G-Zn further suppressed the transfer efficiency of its Cd to the plant. No apparent Cd x Zn interaction could be implicated in the suppression in this experiment, although the additions enhanced Zn accumulation and widened the Zn:Cd ratio in the plant. Further research is needed to determine if the suppression was caused by constituents other than Zn in the fertilizer.