Effect of Soil Cobalt Concentrations on Some Growth Indices of Pinto Bean

Cobalt has not been categorized as an essential plant nutrient, although it has been shown to be useful to some crops. On the other hand, cobalt is discharged on soils as part of some wastes and effluents. The effect of cobalt on the growth of pinto bean has not been studied yet. To evaluate the effect of cobalt on pinto bean, a pot experiment was carried out in 2010 in the Shahrekord University greenhouse with 5 soil cobalt levels (0, 20, 70, 150 and 220 mg Co/kg dry soil) in a completely randomized design. After physiological maturity, leaf area, stem length, root length, stem and root dry weights were measured. Strong linear relationships were observed between soil cobalt concentration and cobalt concentration in root, stem, and leaf and, therefore, the whole plant (p<0.01). Most of the absorbed cobalt was retained in the root. At 20 mg Co/kg soil, all growth indices, including allometric coefficient, improved (p<0.01). All growth indices were negatively affected in a linear manner with increasing soil cobalt concentration. Relative growth inhibition initially decreased but increased afterwards with higher soil cobalt concentrations. Overall, a tolerance threshold of 70 mg Co/kg of dry soil (or 350 mg Co/kg dry weight of aerial parts) was estimated for pinto bean.