Traditional farmers supplied the needed nitrogen in two ways: by recycling any available organic materials (straw, stalks, leaves, human and animal waste) and by rotating grain or oil crops with leguminous plants (cover crops such as alfalfa, clovers, and vetches; and food crops such as soybeans, beans, peas, and lentils). These plants are able to supply their own nitrogen because bacteria attached to their roots can “fix” nitrogen (convert it from the inert molecule in the air to ammonia that is available to growing plants) and they also leave some of it behind for the following grain or oil
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