Crop Rotation on Organic Farms: A Planning Manual (Mohler and Johnson, free download here) is the result of an organized effort to capture local knowledge of popularly-nominated expert organic vegetable growers in the Northeast.
It was interesting to listen to the seminar speaker describe the impetus and main findings of this study. Textbook descriptions of crop rotation apparently tend to be rigid and idealistic - so the authors made an attempt to describe them in a more intuitive fashion that will facilitate on-the-fly rotations that allow for unpredictable weather and markets.
Crop rotations are key to preventing the buildup of host-specific pests and pathogens, while evening out soil nutrition when chemical applications aren't an option. Completely stripped down, they basically recommend that you keep a given plant family (e.g. tomatoes-potatoes-peppers-eggplants, legumes or melons-squash-cucumbers) in a given plot for only 1 in every 4 years. Grasses can be cycled more quickly since our local pests and pathogens here tend to be specific at the Genus instead of Family level. They also give common sense advice on how to prioritize which crops should go in the best fields.
I was shocked to hear that these farmers have virtually NO weeds in their fields. Apparently they pull this off with zero-tolerance for weed seed contamination in crop seed or compost and summer fallows (e.g. no bare dirt), finished off with hand pulling. On most farms, weeds are the No. 1 problem - so this is pretty impressive management!
I was also impressed by the extensive use of intercropping.
During the Q&A, we learned that overcomposting is a major problem (that doesn't help keep weeds down and is at times absurdly unsustainable). I guess being surrounded by horse farms looking to get rid of tons of manure is tempting towards overuse...
These farmers don't rotate animals onto their fields since they don't want the soil (that they work so hard to fluff up) to get compacted. They may occasionally bring in chickens or ducks (to eat slugs), but having to deal with fences to keep the animals in and predators out is rarely worth the effort.
One audience member asked if today's expert organic farmers are any better than their counterparts in the Middle Ages. The speaker mused that since all of this knowledge simply arose from lifetimes of local observation, that they probably aren't.
(Outside of new technologies of course!)