Use of Co speciation and soil properties to explain variation in Co toxicity to root growth of barley ( Hordeum vulgare L.) in different soils

The influence of soil properties on the bioavailability and toxicity of Co to barley ( Hordeum vulgare L.) root elongation was investigated. Ten soils varying widely in soil properties were amended with seven doses of CoCl 2. Soil properties greatly influenced the expression of Co toxicity. The effective concentration of added Co causing 50% inhibition (EC 50) ranged from 45 to 863 mg kg −1, representing almost 20-fold variation among soils. Furthermore, we investigated Co toxicity in relation to Co concentrations and free Co 2+ activity in soil solution. The EC 50 values showed variation among soils of 17- and 29-fold, based on the Co concentration in soil solution and free Co 2+ activity, respectively. Single regressions were carried out between Co toxicity threshold values and selected soil properties. Models obtained showed that soil effective cation exchange capacity (eCEC) and exchangeable calcium were the most consistent single predictors of the EC 50 values based on soil added Co. Soil eCEC and exchangeable Ca were found to be the best predictors of the toxicity threshold values of Co to barley root growth on different soils.