I have to add heavy metal spiked like Pb, Cr, Cd, As, and Ni in soil (in crates) more than the permissible limit. My 1st question is wether i have to add combination of all or individually? 2nd question is any single material available for supplying all heavy metal? Or individual meterials only? 3rd question is how to prepare 5ppm of heavy metal for 50kg of soil?