Plowing a garden in fall may be best as others have explained, unless you want to take advantage of some "organic" rotation. Most of the plowing I do is of old white-clover plots for spring planting of corn. In this special case, I like spring plowing for two reasons. First, no ground is less attractive to deer than plowed. This way I have all those acres of deer-attracting clover around thru the fall, winter, and early spring making for much better hunting than if I fall-plowed. Second, keeping the clover growing in the fall and the early spring stores up some more nitrogen, reducing the ammount that must be added later for a good corn yield. I prefer spring plowing of even my garden (mostly sweetcorn and pumpkins) for this second reason. To use this method, you have to move your garden every year, on a 3-5 year rotation, with white clover grown on the off years. If you need to plant your garden in the same spot each season, then fall plowing would be best. I think I have found a little better way to do it, and my garden has done much better since I started "moving it around".