THEORY OF SOIL FERTILITY 193 



This method, if care be taken to supply an 

 abundance of all the necessary constituents, 

 may result in a complete, though perhaps not 

 an economical, feeding of the plant, since it 

 assumes that a plant that contains a larger 

 amount of one constituent than of another 

 requires more of that constituent in a ferti- 

 lizer than of the others. It does not take 

 into consideration the fact that the plant which 

 contains a larger amount of one element than 

 of another may possess a greater power of 

 acquiring it than one which contains a smaller 

 amount." 



But, aside from the possibility of maintain- 

 ing the fertility of the soil by the addition of 

 chemical nutrients, the question remains, how 

 long will the soil continue to produce crops, 

 by means of its innate fertility? Now, the 

 liberation of available plant food by weather- 

 ing is comparable to rate of interest on a trust 

 fund in an interest-paying bank only in a 

 measure. A trust fund in a bank is actually 

 at work producing additional capital, which 

 it pays as interest. The trust fund in the soil 

 on the other hand is at work liberating a por- 

 tion of the capital itself. The statement that 

 the American farmer is "mining" the soil is a 

 familiar one. It comes from this conception 



