IV] FERTILITY LIMITS 45 



Thus each soil may vary in composition and 

 therefore in fertility 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 equili- 

 brium 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," according 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 fer- 

 tility level is not wholly a question of percentages of 

 nitrogen, carbon, etc. At its highest level the soil 

 possesses a good physical texture owing to the floc- 

 culation 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 unsatisfactory. Much 

 calcium carbonate is also lost during the process: 

 and when this constituent falls too low the soil 

 becomes "sour" and unsuited to certain crops. 



The simplest system of husbandry is that of 



