A 20x20 foot food plot is not very big. Considering this, I would suggest keeping the natural growth in check throughout the summer and about 3 weeks prior to your hunting season plant a mixture of med red clover (or equivalent), spring oats, and winter rye grain.
It will take them a little while to find this small plot but they may stop and take a few bites while headed to their feeding destination or bedding area. You are using this as a kill plot correct? Your biggest hurdle will be getting enough light. Can you hinge or cut any trees around to let in more light?
Either way amend your soil with lime and add some tripple 13.