Plant toxicity testing to derive ecological soil screening levels for cobalt and nickel

Phytotoxicity tests were performed to set ecological soil screening levels for cobalt (Co) and nickel (Ni) following the American Society for Testing and Materials international E1963-98 Standard Guide for Conducting Terrestrial Plant Toxicity Tests. Two soils (a modified artificial soil mixed with 5% organic matter, pH 5.01, and a native riverine sandy soil with 0.1% organic matter, pH 6.3) were treated with cobalt(II) chloride or nickel chloride and allowed to age for four weeks before initiating tests. Alfalfa, barley, radish, perennial rye, and brassica were used to determine the appropriate range of concentrations and to select the most sensitive plant species for definitive tests. The tests were designed to have one to three test concentrations below the 20% effects concentration (EC20), and five to six test concentrations above the EC20. Definitive tests for each chemical used two soil matrices, three plant species, and replicates at 10 nominal concentrations, including negative control. Soil chemical concentrations were determined before planting and on completion of the phytotoxicity tests. Threshold responses interpreted as the EC20 for each species endpoint were calculated from regression analyses. The geometric mean of the EC20 values (excluding emergence, mortality, and nodule numbers) for each species resulted in values of 30.6 mg/kg for Co and 27.9 mg/kg for Ni.