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A biological model of nitrogen in agriculture is specified for early modern England and used to analyze the growth in grain yields from the middle ages to the industrial revolution. Nitrogen-fixing plants accounted for about half of the rise in yields; the rest came from better cultivation, seeds, and drainage. The model highlights the slow chemical reactions that governed the release of the nitrogen introduced by convertible husbandry and the cultivation of legumes. However efficient were England's institutions, nitrogen's chemistry implied that the English agricultural revolution would be much more gradual than the Green Revolution of the twentieth century.