1 ppm = 1 mg/kg. So 5 ppm = 250 mg/50kg. Assuming you might want to add 250 mg of a specific or each heavy metal, adjustments would be made for the compound used, and whether you wanted to make a solution and add it to the soil to be mixed and dispersed. This mixing and dispersal process would be difficult with some soil types, such as a crate of clay.

Then one would have to know if you wanted a uniform mixture. That might not be easy to do. Whether to add in combination or singly would depend on your study and Statistical analysis plans. If the contaminants are not uniformly dispersed, then taking subsamples may not be representative. An inexpensive cement mixer may help with dispersing the chemical(s) in the mix, and then dump back into crates. Since you would be dealing with hazardous materials, best to have an analytical chemist and possibly statistical review your study plans and safety protocols. It is much easier to develop a plan and anticipate any problems, get advice from other specialists, than forging ahead, doing the work and finding out later the study is flawed.

William F. Hansen thankyou so much sir.