get a ten gallon drum you'll do fine in that, you roots will have more room than they actualy need. but all and all the answer is yes the clay will kill your plants, either dig a REALLY BIG hole or use a contianer, if the clay dosn't kill em it'l make your buds tase "dirty" i had the same clayish problem with a sativa plant i was raising near a riverbed and it lived but the bud literaly tasted like dirt and a plant from the same seedstock grown in my potting mix tasted great.the soil by the riverbed was a a light red clay (grandcanyon-ish) and all i did to the plant was fertilize it normaly and tied it to stakes when it got about 17ft tall...turned out all the riverbed weed was good for in the end was making hash oil!