FEETILITY 165 



In practice there is both gain and loss, but the former 

 exceeds the latter, and so, each time the cycle is completed, 

 the capacity of the soil for the production of crops is 

 increased. 



Effect of Removing Crops from the Land. The quantity 

 of plant foods absorbed from the soil by a growing crop 

 is considerably greater than the quantity that is converted 

 into an available state in the same time, and, if the crops 

 are removed from the land, the productiveness is neces- 

 sarily diminished. It is obvious that a system of farming 

 which tends to reduce the fertility of the soil must sooner 

 or later come to a stop, and that, if farming is to be carried 

 on .continuously, the system must be modified so as to 

 maintain or, if possible, to increase the fertility. 



Maintenance of Fertility. There is thus an apparent 

 dilemma. On the one hand, it is useless to grow crops unless 

 they can be removed from the land ; on the other, if the 

 crops are removed the fertility is reduced. But it is pos- 

 sible to pursue a middle course, namely, to withdraw from 

 the cycle, i.e., to permanently remove from the land only 

 so much produce as corresponds to the quantity of plant 

 food that becomes available in the same time. On cul- 

 tivated land the change is greatly accelerated by tillage, 

 and there is, therefore, a larger margin to draw upon. The 

 amount of produce annually withdrawn from the cycle 

 is reduced by subjecting the land to occasional or 

 periodical fallow, by laying it down to grass, and by 

 restoring a portion of the crops. To obtain a profitable 

 return when the amount of produce that can be safely 

 withdrawn is so limited is not a simple matter, and deserves 

 more careful consideration. 



Fallow. Probably at one time it was customary to til] 

 the land and to remove the crops until it ceased to give 

 a profitable return for the labour expended upon it, and then 

 it was neglected. When it was found that the fertility of 



