Some metals and metalloids such as copper (Cu), manganese (Mn), nickel (Ni), zinc (Zn), iron (Fe), boron (B), and molybdenum (Mo) are essential for most plants but toxic when supplied in supraoptimal quantities
meanwhile, the biological role of cadmium (Cd), mercury (Hg), lead (Pb), and arsenic (As), with rare exceptions, is unknown, and they are toxic for plants even at fairly low concentrations [...].