i itrogen is in a class alone, for in most agriculture its supply governs the IN yield of crops that have enough water.' The implications of this truth for the history of agriculture have barely yet been recognized, although it was long ago established by Lawes and Gilbert in their famous controversy with Liebig. The whole problem of nutrient supply (at least before the advent of commercial fertilizers), despite its critical bearing upon the evolution of agricultural productivity and so of the economy in general, has received scant attention from historians. When they do discuss it, they do so, it is hardly an exaggeration to say, in terms that most often suggest that Liebig, Gilbert, and their successors in the development of agricultural science had not existed. This primitive approach to the problem, the corollary of its general neglect, is particularly striking in the historiography of the complex changes, conveniently referred to as the Agricultural Revolution, which took place on the European Continent between about 175o and i88o. During this period there was a marked increase in the cultivation of leguminous crops, the only variable on the supply side of the nitrogen economy subject to human manipulation at that time. How great was the increase? What effect did it have upon the overall supply of nitrogen in the system and on the level of production? How important a constituent was it of the Agricultural Revolution? Little attempt has been made to answer these questions, beyond the occasional reference to the nitrogen-fixing capacity of legumes. The issue, central though it evidently is to an understanding of the increase in agricultural output achieved during the period, has been confused and lost sight of in an indiscriminate emphasis upon the importance of fodder crops generally and the production of manure tout court; and a failure to distinguish between legumes, which add to the quantum of nutrients in the system, and roots and potatoes, which on the contrary diminish it. In seeking to explain the rise in output per hectare, historians are customarily content even now to invoke the old questionbegging formula so dear to the agronomists of the later eighteenth century: more fodder crops equals more livestock equals more manure equals higher yields. This cercle de l'economie rurale, as the Feuille du Cultivateur called it in I 790, was by no means necessarily a virtuous one.2 Whether it was so depended essentially upon whether the fodder crops were leguminous. Furthermore, its beneficial