I think that's not so. For one thing, a lot of "city dirt" is contaminated with chemicals. The dirt that's supposed to be good for you, is good for you because of it's organic contaminants, not chemical ones.

Furthermore, the specific organic contaminates that make the most difference, apparently, are parasitic worm larvae, which are not so likely to be found in the city, as they come from animal feces. Of course, there are dogs and cats in the city, but I'm more comfortable with cow crap than dog crap, myself, as far as ingesting it. Plus, people worm their pets.

All matter is in the form of chemicals, including everything possibly called organic.