Here's a new thread that may talk about starting up a new bed:
The thread "Tips for starting a new garden bed" in
All Things Gardening forum
They expanded on the idea of "pile compost makings on top of your bed-to-be" by using the "official" names "lasagna gardening" and sheet composting. That is the best way to look up the details.
The way that I think about compost is that the only hard part is
collecting enough organic stuff to use. Once you have lots of greens and browns, almost anything you do with them will enrich your soil. The main advantage to getting fancy with compost is that you can make it break down faster.
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. And it's easy to put some hoops and plastic film over a narrow bed, protecting crops from Spring cold snaps and early Fall frosts. Or to add summer shade covers.
But watering a narrow bed efficiently is harder, unless you have dripline, T-tape or soaker hoses.. And a narrow bed has more edges for grass to invade from.
