SOIL SANITATION 291 



The belief that the process of producing 

 crops on a soil causes the formation of toxins 

 is not new. De Candolle suggested, in 1832, 

 that plant roots excrete a poison which sooner 

 or later would render any soil sterile if sub- 

 jected to single-cropping. (Physiologic 

 Vegetale, Paris, 1832.) Liebig accepted this 

 explanation as the true one at first, but later 

 began his researches in the mineral require- 

 ments of plants and abandoned his original 

 position. Once his Theory of the Mineral Re- 

 quirements of Plants became adopted the sug- 

 gestion of de Candolle was forgotten. Only 

 when American scientists turned their atten- 

 tion again to the part played by the organic 

 elements of the soil was this suggestion exam- 

 ined in detail or with any seriousness of 

 purpose. 



Even to-day our own scientists do not ac- 

 cept de Candolle's suggestion literally. They 

 admit they have isolated substances from soils 

 following plant growth, but they are not ready 

 to admit that these substances are the result 

 of root excreta. In several instances the facts 

 seem to point to that explanation. In others, 

 however, the origin of these substances is much 

 more complex. They may be the result of 



