THE PRINCIPLES OF MANURING 181 



process of replacing plant food previously abstracted 

 from the soil and which cannot be removed without de- 

 preciating the fertility ; or it may be an addition of plant 

 food made for the purpose of increasing the productiveness 

 of the soil beyond its natural capacity. The functions 

 of manures are therefore twofold to maintain fertility 

 and to increase it. 



Eestoration is effected chiefly by farmyard manure, 

 and that is its principal function. So long as the con- 

 stituents of farmyard manure are all derived from the 

 soil it cannot possibly increase fertility; at best, it 

 can only maintain it, unless the sale of produce is 

 restricted to less than the equivalent of the quantity of 

 plant food that is converted into an available state during 

 the period of production ; if the sale of produce were so 

 restricted it would be difficult to obtain a profitable 

 return. The fertility of the land may, however, be in- 

 creased by the use of farmyard manure if it is wholly or 

 partially derived from external sources. For example, 

 if it be not produced from the soil but purchased from 

 town stables, etc., as is usually the case in gardens, or 

 if it contain fertilising ingredients derived from oil 

 cake or other purchased foods consumed by animals. 



As it is generally impossible to purchase farmyard 

 manure, addition can, as a rule, only be made in the form 

 of the so-called artificial manures. To increase fertility 

 not to maintain it is therefore the principal function 

 of these substances. It is obvious, of course, that if, 

 for any reason, the farmyard manure were withheld, an 

 equal quantity of plant food could be restored to the 

 soil in the form of artificial manure. Whether it would 

 serve the purpose equally well is a matter for further 

 consideration. 



In general, the plant foods abstracted from the soil in 

 the course of the rotation are restored in the farmyard 



