You are only going to have success with a physical barrier. Consider rat bait boxes, as well keeping up as the traps, but the rats must have a food source next door, so they will never be able to be eradicated by you alone. One rat will destroy your whole crop.
Make a cage out of coated chicken wire. It had to be a box shape, covered on the top as well, it also has to extend outward 1 foot on the ground from the base of the "box."
Visualize a cardboard box with the open side on the ground and the flaps extended on the ground.