Here's a new thread that may go into starting up a new bed:
The thread "Tips for starting a new garden bed" in
All Things Gardening forum
P.S. I guess you would not be growing much corn in a 20' x 20' plot.
You might consider whether it's possible to make it 4' x 100' or even 3' x 140'.
Or maybe 5 beds, each 4' x 20', with a 12" or 18" walkway between each. You can shovel soil from the paths onto the beds, giving more root zone in the beds. if the paths are excavated down to subsoil, it MIGHT discourage weeds from sprouting in the paths.
The narrower it is, the easier it is to reach the whole bed without walking on the soil, which leaves your soil much less compacted. If there are no unused soil between rows, there's much less to weed.
But watering is harder. And grass has more edges to invade from.
