The Economics of Exhaustion, the Postan Thesis, and the Agricultural Revolution

The Economics of Exhaustion, the Postan Thesis, and the Agricultural Revolution

By Gregory Clark

The Journal of Economic History, Vol. 52, No. 1. (1992)

Abstract: The Postan thesis is that medieval agriculture had low yields because there was insufficient pasture to keep the arable land fertile. This argument (and variants of it) has become an orthodox technological explanation for low preindustrial yields. Yet the thesis, on its face, implies that early cultivators were ignorant, irrational, or completely custom bound. This article develops a revised Postan thesis, in which medieval cultivators knew that pasture restored fertility but were unwilling to employ it. Impatience made this way of increasing yields unattractive because it required large capital investments in the soil nitrogen stock.

Introduction: Preindustrial Europe was impoverished and thinly populated, and it remained so for a surprisingly long time. As the major economic activity was producing food, the key to the growth of total income, and of income per capita, was increasing agricultural output. Medieval grain yields, for example, were astonishingly low. In many years the yield was only two or three grains for each grain planted. In southern England, in the years before the Black Death, I estimate that the net yield per acre from cultivated land was only equivalent to 4 bushels of wheat, compared with 13 circa 1850. The low yields per acre could sustain only a small population, even at the subsistence consumption standards of the preindustrial era.

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