64 THE FERTILITY OF THE SOIL [ch. 



CHAPTER V 



THE RAISING OF THE FERTILITY LIMIT 



We have seen in the previous chapters that the 

 fertility of a given soil may lie anywhere between 

 two limits : the liigher limit being attained when the 

 land is allowed to remain with an undisturbed vegeta- 

 tion of grass and clovers, and the lower when the 

 land is perpetually under the plough, producing 

 nothing but cereal crops and receiving no manure to 

 counterbalance the various losses. We have further 

 seen how, by a judicious system of husbandry, it is 

 possible to maintain arable land somewhere near the 

 higher fertility limit by arranging for recuperative 

 periods in grass and clover, and systematically adding 

 manurial substances to replace whatever may be lost. 



The higher limit beyond which the fertility of the 

 soil as it stands cannot be pushed, is set by the nature 

 of the soil, its position in respect to water supply, 

 climate, etc. But it is often possible to change these, 

 and when this is done the fertility is no longer tied 

 down to the old limit but rises to a new one set by 

 the new conditions. This process is of course funda- 

 mentally different from the case we have just dealt 

 with, and is in practice so much more costly that the 



