SOIL SANITATION 279 



but it cannot be grown successfully every year 

 without a quick drop in productivity. Here 

 is a factor in fertility, or the lack of it, in 

 which "plant food" as such plays no part. 



In attempting to explain the peculiar be- 

 havior of this pot of soil, all of the usual fac- 

 tors could be set aside. The soil was rich in 

 minerals, its organic content was maintained, 

 it was well watered, and the temperature was 

 at the optimum for the production of these 

 crops. Going back to the analogy of the tired 

 laborer with poison in his muscles, it was sug- 

 gested that this soil was tired. It was given 

 no rest between crops. In nature, seasons in- 

 tervene, to give the recuperative agencies of 

 the soil a chance to work. Even if such 

 agencies were present here, they were given 

 no opportunity to work. 



The experimenters set about looking for the 

 poison of fatigue in this pot of soil. To con- 

 centrate this "poison," supposing it to exist, 

 they forced the pots to grow several crops of 

 wheat and cow-peas in succession. Then they 

 took the soil and subjected it to analysis. They 

 isolated crystalline substances from these soils 

 that had grown tired of growing wheat and 

 cow-peas. Searching for the same substances 



