A COMPARISON OF TESTS FOR EXTRACTABLE COPPER AND ZINC IN METAL-SPIKED AND FIELD-CONTAMINATED SOIL

Abstract

Common soil test methods were compared with 0.01 M CaCl2 extraction to determine their relative abilities to extract and estimate phytoavailability of Cu and Zn. In aged metal-spiked soils, all soil tests evaluated (Mehlich 3, diethylenetriaminepentaacetic acid [DTPA], Morgan, modified Morgan, CaCl2) showed a linear relationship of extractable to total Cu and Zn for both soil types studied. The fraction of total Cu and Zn extracted by aggressive tests (Mehlich 3, DTPA) was much higher than the fraction extracted by CaCl2, with the Morgan tests being intermediate. Although all extraction methods revealed Cu and Zn to be more available in the coarse-textured soil than the fine-textured soil, this texture effect was greatest for the least aggressive test (CaCl2) and least for Mehlich 3. The texture effect on soil test Cu and Zn was also reflected in bioavailability, with greater plant tissue concentrations of both metals from the coarse-textured soil in a soybean assay. Although all soil test methods provided similarly strong correlations to plant tissue concentrations of Zn for soybeans grown in the two soils, the less aggressive soil tests seemed more reliable as predictors of Cu uptake. The efficiency of Cu and Zn extraction from field-contaminated soils was much lower than that from laboratory-spiked aged soils. For Mehlich 3 and DTPA tests, Cu and Zn in field-contaminated soils were less extractable by a factor of about 2 compared with the spiked soils. For less aggressive tests, the difference in extractability was even greater. This study suggests that soil extraction methods removing smaller pools of Cu and Zn are more responsive to soil properties affecting chemical lability of the metals. Therefore, chemically nonaggressive neutral salts may be the most appropriate extractants where phytotoxicity is the concern in metal-contaminated soils.