Phytotoxic Effects of Cu and Zn on Soybeans Grown in Field‐Aged Soils: Their Additive and Interactive Actions

A field pot experiment was conducted to investigate the interactive phytotoxicity of soil Cu and Zn on soybean plants [ Glycine max (L.) Merr.]. Two soils (Arkport sandy loam [coarse‐loamy, mixed, active, mesic Lamellic Hapludalf] and Hudson silty clay loam [fine, illitic, mesic Glossaquic Hapludalf]) spiked with Cu, Zn, and combinations of both to reach the final soil metal range of 0 to 400 mg kg −1 were tested in a 2‐yr bioassay after 1 yr of soil‐metal equilibration in the field. The soluble and easily‐extractable fraction of soil Zn (or Cu), estimated by dilute CaCl 2 , increased linearly in response to the total Zn (or Cu) added. This linearity was, however, strongly affected where soils were treated with both metals in combination, most notably for Zn, as approximately 50% more of soil Zn was extracted into solution when the Cu level was high. Consequently, added Zn is less likely to be stabilized by aging than added Cu when both metals are present in field soils. The predictive model relating soil metal extractability to plant Zn concentration also revealed a significant Cu–Zn interaction. By contrast, the interaction between the two metals contributed little to explain plant Cu uptake. The additive action of soil Cu and Zn was of considerable importance in explaining plant biomass reduction. This work clearly demonstrates the critical roles of the properties of the soil, the nature of the metal, and the level of other toxic metals present on the development of differential phytotoxicity due to soil Cu and Zn.