I tend a large 10th floor center-of-city balcony for my employer, and there are 2 built-in concrete planters (roughly 9' x 9'/18" deep) full of soil that has been here probably since the building was built (late 70s more or less). For the past 4 or 5 years, I've grown ornamentals (organically). I'm about to redo it for the season, and am intrigued at the idea of using one for edibles. I expect there must be considerable lead from traffic below, and heaven knows what else (luckily, we get no rodents as they cannot scale 10 stories of smooth concrete).
How do you urban growers deal with the risk of contaminated soil for edibles? Replacing the soil completely is not an option. Am I just overcautious or would I really need to have it tested? Illness as a result of toxicity is unwanted for all reasons, legal liability of my employer not the least of them. I'm considering pots, but there is not much extra space for enough to make it worthwhile.