Ed,
Up at our cabin much of the soil is beach sand, black dirt only as deep as the grass roots, everything else seems to percolate down through the sand. I've dug IHs that still are copper colored there. Here in Wisconsin mainly hunting a river valley with very friendly soil, but if I travel east very far get into a more clay based soil, and copper is often toast.
My GF threw some crusty cents that I had given up on in some vinegar, and lo and behold most of the crusties cam off without completely killing the coin. Will experiment more; will tumble them later. These were green grungy discs impervious to H2O2, Tried electrolysis on similar ones with bad results. Who knew?
One nice thing about the nice soil is that I have an IH collection that includes many coins lost very early in their lives, many EF or almost uncirculated.
Chris