2oi] THE FERTILITY OF THE COMMON FIELDS 45 



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 re- 

 garded as fairly stationary. 



When, however, the virgin soil is broken up by the plough 

 and brought into cultivation the native vegetation and the crop 

 are alike removed, and therefore the sources of gain are con- 

 siderably reduced. The losses, on the other hand, are much 

 intensified. Rain water more readily penetrates, carrying dis- 

 solved substances with it : biochemical decompositions also pro- 

 ceed. In consequence the soil becomes poorer, and finally it is 

 reduced to the same level as the rate of gain of nitrogenous 

 matter. A new and lower equilibrium level is now reached 

 about which the composition of the soil remains fairly con- 

 stant; this is determined by the same factors as the first, i. e. 

 the composition of the soil, climate, etc. 



Thus each soil may vary in composition and therefore in fer- 

 tility between two limits : a higher limit if it is kept permanently 

 covered with vegetation such as grass, and a lower limit if it is 

 kept permanently under the plough. These limits are set by the 

 nature of the soil and the climate, but the cultivator can attain 

 any level he likes between them simply by changing his mode 

 of husbandry. The lower equilibrium level is spoken of as the 

 inherent fertility of the soil because it represents the part of the 

 fertility due to the soil and its surroundings, whilst the level 

 actually reached in any particular case is called its condition or 

 " heart ", the land being in " good heart " or " bad heart ", ac- 

 cording as the cultivator has pushed the actual level up or not; 

 this part of the fertility is due to the cultivator's efforts. 



The difference between the higher and lower fertility level 

 is not wholly a question of percentage of nitrogen, carbon, etc. 

 At its highest level the soil possesses a good physical texture 

 owing to the flocculation of the clay and the arrangement of the 

 particles : it can readily be got into the fine tilth needed for a 

 seed bed. But when it has run down the texture becomes very 



