I thought this was a really detailed and fascinating article about how
a self-contained 6-acre organic farm in PA succeeds in maintaining
fertility. I've actually watched Cornell University's small farm
program you-tube videos about this farm, but focusing more on their
pig husbandry.

http://newfarm.rodaleinstitute.org/features/1204/nordell/index.shtml

an excerpt:
The Nordells only compost and apply manure generated by their own four
horses. Compared to many market gardens, the rate of application per
acre is low. But unlike traditional market gardeners, the Nordells
apply compost to cover crops, not their cash crops. The cover crops
contribute to soil health and in turn fertilize the cash crops. This
also avoids flushes of weeds during cash crop years due to nutrient
excesses or imbalances.