THEORY OF SOIL FERTILITY 203 



of the general report of the Commission before 

 its untimely end. However, other organiza- 

 tions and individuals have gone further, and 

 for one who cares to figure on our soil resources 

 according to Liebig there is ample material, 

 gathered by competent men. 



Dr. Cyril G. Hopkins of Illinois is among 

 those who have reduced the exhaustion of soil 

 fertility to a matter of years. We quote him 

 because of his undoubted position as an edu- 

 cator in the agricultural sciences, and his 

 opinion should carry weight. In addition he 

 is the spokesman of that overwhelming ma- 

 jority among our scientists who stand staunch 

 in their belief that the theory of Liebig is in- 

 vulnerable, so far as mineral requirements and 

 resources are concerned. 



In a recent article (Country Gentleman, 

 Vol. 78, No. 3) he discusses the subject briefly. 

 First he enumerates the important plant foods. 

 Carbon and oxygen, he says, are taken by the 

 plant from the air in the form of carbon 

 dioxide, through the breathing pores, located 

 chiefly on the under side of the leaves. Hydro- 

 gen is derived by the plant from the water it 

 absorbs through its roots. These three ele- 

 ments constitute the larger part of the mature 



