[[36]] Cf. infra, p. 82.
[[37]] Tawney, op. cit., p. 220, note 1.
[38] Infra, p. 78, 81, 98-9.
CHAPTER II
The Fertility of the Common Fields
Up to this point attention has been given chiefly to the theory that the enclosure movement waxed and waned in response to supposed fluctuations in the relative prices of wool and grain, and it has been found that this theory is untenable. It is now necessary to consider more closely the true cause of the conversion of arable land to pasture—the declining productivity of the soil—and the cause of the restoration of this land to cultivation—the restoration of its fertility.
The connection between soil fertility and the system of husbandry has been explained by Dr. Russell, of the Rothamsted Experiment Station:
Virgin land covered with its native vegetation appears to alter very little and very slowly in composition. Plants spring up, assimilate the soil nitrates, phosphates, potassium salts, etc., and make considerable quantities of nitrogenous and other organic compounds: then they die and all this material is added to the soil. Nitrogen-fixing bacteria also add to the stores of nitrogen compounds. But, on the other hand, there are losses: some of the added substances are dissipated as gas by the decomposition bacteria, others are washed away in the drainage water. These losses are small in poor soils, but they become greater in rich soils, and they set a limit beyond which accumulation of material cannot go. Thus a virgin soil does not become indefinitely rich in nitrogenous and other organic compounds, but reaches an equilibrium level where the annual gains are offset by the annual losses so that no net change results. This equilibrium level depends on the composition of the soil, its position, the climate, etc, and it undergoes a change if any of these factors alter. But for practical purposes it may be regarded as fairly stationary.