44 THE FERTILITY OF THE SOIL [CH. 



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. 



When, however, the virgin land is broken up by 

 the plough and brought into cultivation the native 

 vegetation and the crop are alike removed, and there- 

 fore the sources of gain are considerably reduced. 

 The losses, on the other hand, are much intensified. 

 Rain water more readily penetrates, carrying dissolved 

 substances with it: biochemical decompositions also 

 proceed. In consequence the soil becomes poorer. 

 But the impoverishment does not go on indefinitely : 

 the rate of loss diminishes as the soil becomes poorer, 

 and finally it is reduced to the same level as the rate 

 of gain of nitrogenous organic matter. A new and 

 lower equilibrium level is now reached about which 

 the composition of the soil remains fairly constant; 

 this is determined by the same factors as the first, 

 i.e. the composition of the soil, climate, etc. 



