FEETILITY 177 



nothing. If the animals are fed on nothing but straw 

 the dung will contain nothing but what the straw contains. 

 If the food is enriched by the addition of oil cake, etc., 

 the dung will be enriched by the constituents of the oil 

 cake. The quantity of plant food in the soil may be in- 

 creased by addition either from natural sources or in 

 other ways. It may be diminished by abstraction by crops 

 or by accidental losses. The quantity will remain con- 

 stant only when the additions are exactly equal to the 

 abstractions. A certain minimum quantity of plant food 

 is necessary for the production of a maximum crop; 

 if less than this minimum be present the crop will be 

 correspondingly decreased. 



S.M. 



