EVIDENCE OF HISTORY 261 



dealing with the use of chemicals. The mere 

 fact that a chemical was used in the process of 

 farming a field that proved especially fertile 

 does not necessarily mean that the added pro- 

 ductivity was due solely to the use of that 

 chemical. Liebig himself recognized the in- 

 sufficiency of his theory of mineral require- 

 ments. He comments frequently in his writ- 

 ings on the fact that crops do not respond in 

 proportion to the plant food fed to them; in 

 fact, he notes, in many instances, that soils fall 

 off in productivity even when fed freely when 

 subjected to the same crop year after year. 



What are these experiments worth then to 

 the farmer? 



They are worth nothing, unless he is capable 

 of interpreting them with full knowledge of 

 conditions. Unfortunately, much of the early 

 data is valueless because of the inclination of 

 the experimenters to consider all other con- 

 ditions as fixed and only the mineral plant- 

 food requirement of soils as varying. 



Even when climatic and cultural conditions 

 are identical, the widest range of yields is 

 experienced. The average farmer has no con- 

 ception of the different types of soils, both 

 physical and chemical, which are to be found 



