If you're concerned about the chemical content in the soil that you would be using then look to detox. This is from http://www.dirtdoctor.com/.

Soil Detox

Soil Detox for Contaminated Soil

Digging the soil out out and hauling it off is not the answer. That just moves the problem from point A to point B.

If your soil has been contaminated with heavy metals like arsenic and chromium in treated lumber or creosote in railroad ties, or with lead and arsenic from iron supplements, or if the contamination is from pesticides or petroleum spills, the solution is the same. First, stop the contamination. Second, apply the activated charcoal product from NORIT called GroSafe. It's very fine-textured and must be mixed with water to apply.

The next step is to drench the problem area with the Garrett Juice solution plus orange oil. Use the Garrett Juice formula that's detailed on the web site dirtdoctor.com and add 2 ounces of orange oil or d-limonene per gallon of mix. GroSafe will tie up the contaminants, the Garrett Juice and orange oil stimulate the microbes to eat the contaminants. Liquid molasses is in the Garrett Juice mix but adding dry molasses to the soil at 10-20 per 1000 sq ft will greatly help the decontamination process.

SRS