Personally, I have found more older coins in dryer hard soil then in softer easy to dig soil. Reason, it seems targets sinks faster in softer wet soil. I have dug 3-4 year old coins 7-8" deep in Oregons wetter softer sport fields.
I dug 10-15 wheats from the 40s/50s in a Sacremento yard less then 1" deep an not more then 3" deep. I dug some 20/30s relics out of the same yard at the deepest of 7-9" in the softer more watered area of the yard.
Yep, harder and like Bman says....OLD really OLD sites