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method of agricultural organization introduced in Europe in the Middle Ages and representing a decisive advance in production techniques. In the old two-field system half the land was sown to crop and half left fallow each season; in the three-field system, however, only a third of the land lay fallow. In the autumn one third was planted to wheat, barley, or rye, and in the spring another third of the land was planted to oats, barley, and legumes to be harvested in late summer. The legumes (peas and beans) strengthened the soil by their nitrogen-fixing ability and at the same time improved the human diet.