Townshend’s rotation — like the ones George Washington and Thomas Jefferson used — included clover, wheat, other small grains and turnips, which made good winter food for sheep and cattle. My uncle grew no turnips, but he, like all his neighbors, was using his own version of the four-crop system, at the heart of which was alfalfa.

Getting to the four-crop rotation wasn’t easy, historically speaking. The Romans knew about crop rotation, but by the Middle Ages, farming was based on the practice of letting the land lie fallow, unplanted — resting it, in other words. The purpose of that practice, like crop rotation itself, is to prevent the soil from becoming exhausted when the same crop is sown over and over again. In early American agriculture, only sophisticated farmers like Washington and Jefferson were using crop rotations in their fields. There was simply too much good land available. It was too easy to farm a piece and then move on when the soil was depleted.

In one sense, that is still how modern agriculture works. You look to the future and discard the past. A modern rotation includes only corn, soybeans, fertilizer and pesticides. Whatever you may think about genetically modified crops, the switch to those varieties has driven the rush to the two-crop system. Those crops are designed to tolerate the presence of herbicides. The result is that farmland has been inundated with glyphosate, the herbicide genetically modified crops are engineered for.

The very structure of the agricultural system, as it stands now, is designed to return the greatest profit possible, not to the farmers but to the producers of the chemicals they use and the seeds they plant. And because those chemicals depend on fossil energy, the entire system is inherently unsustainable. What farmers used to return to the soil in the form of labor and animal manure — not the toxic kind you find in livestock confinement systems — they now must purchase, just the way they buy diesel for their tractors.