I grow pole beans along a chain link fence in VERY HARD CLAY soil. I've dug out about 5" deep of clay (by 5" wide) and replaced it with potting soil and the pole beans grow very well. When I dig them up at the end of the season, it appears that their roots never extended deeper nor wider than the 5" of potting soil.

Thanks Rob: I don't have my own pc for the last month /still waiting for it. in the meantime my pole beans are about 3' tall but as I just posted a message, some of the leaves are getting white markings on the outer edge, was wondering what they were.