Zinc and copper toxicity in peanut, soybean, rice, and corn in soil mixtures

Applications of zinc (Zn) and copper (Cu) at excessive rates may result in phytotoxicity. Experiments were conducted with mixtures of soils that were similar except for their Zn and Cu levels. The critical toxicity levels (CTL) in the soils and plants for these elements were determined. Peanut (Arachis hypogaea L.), soybean [Glycine max (L.) Merr.], corn (Zea mays L.), and rice (Oryza sativa L.) were the crops grown. One soil mixture had Mehlich 3-extractable Zn concentrations up to 300 mg dm(-3) with no corresponding increase in soil Cu; two soil mixtures had soil Zn concentrations up to 400 and 800 mg dm(-3) with a corresponding increase in soil Cu up to 20 and 25 mg dm(-3), respectively; and four soil mixtures had no increase in soil Zn, but had Mehlich 1-extractable Cu concentrations from 6 to 286 mg kg(-1). Under a given set of greenhouse conditions, the estimated Mehlich 3-extractable Zn CTL was 36 mg d(m-)3 for peanut, 70 mg dm(-3) for soybean, between 160 and 320 mg dm(-3) for rice, and > 300 mg dm(-3) for corn. No soil Cu CTL was apparent for peanut or soybean, but for corn it was 17 mg dm(-3) and for rice 13 mg dm(-3). With different greenhouse procedures and the Mehlich 1 extractant, the soil CTL for rice was only 4.4 mg kg(-1). Therefore, peanut and soybean were more sensitive to Zn toxicity, whereas corn and rice were more sensitive to Cu toxicity. Plant Zn CTL for peanut was 230 mg kg(-1), while that for soybean was 140 mg kg(-1). Copper appeared to be toxic to corn and rice at plant concentrations exceeding 20 mg kg(-1).