@GG. I did not buy Miracle Grow like the OP. I bought the Vigoro brand and I complained ☺️ that was not a great mix. I did go back and get Miracle Grow organic potting soil when I needed more and was pleased with that product. I never buy a moisture retentive soil. I have to buy the brands that are offered at stores. Miracle-Gro is a very popular brand and sells so that is the soil stores buy. I have not been able to get to a garden center this year. Home Depot is the only place to shop right now for me. I do mix perlite and pine mulch into my soil to help it drain better.

Dan, my only concern about the industrial contamination issue is where you said 'you have no idea where it's from' right after you said 'No question that a lot of this material is from brownfields.' How do you know that?

Granted I live in mostly rural central Missouri so it may be different in urban areas, but the bagged products here come from river bottom fields, river dredging (OK that's potentially a risk as well) and places like that. Not saying it never happens, and it would certainly be an interesting area for research. Shoot, a person with an XRF unit could screen bags of stuff right in the store for metals. Great science fair or grad student project.

In my locale, there is no question that lots of urban residential areas were once industrial sites. I never said those areas were necessarily contaminated. But that's true. The word "brownfield" implies not just industrial use, but pollution.