ORGANIC CONSTITUENTS OF THE) SOU, SOLUTION 97 



there is maintained in them an ample supply of mineral plant 

 nutrients, and they are easily renovated by good absorbers. 

 These facts find a more satisfactory explanation as being due 

 to the production in the nutrient medium of deleterious organic 

 substances originating in the growing plant itself. This idea 

 seems to have been advanced first by De Candolle, in 1832,! to 

 account for the beneficial results obtained by employing a 

 rotation of crops. It appears to have been held by Liebig at 

 one time, although he subsequently abandoned it in favor of 

 the view that the benefits of a crop rotation are due to the several 

 crops requiring different proportions of mineral nutrients, and 

 that the disturbance of the balance in the soil produced by one 

 crop is not unfavorable to the growth of some other crop. 

 Although lacking direct experimental confirmation, this latter 

 view of Liebig's has long prevailed among agricultural inves- 

 tigators, partly by reason of his authority, partly by reason of the 

 dominance of the plant-food theory of fertilizers, and partly 

 by reason of the fact that the ideas of De Candolle as originally 

 advanced included certain errors soon detected. The trend of 

 recent investigations has been distinctly in favor of a modified 

 form of the view of De Candolle. It has been recognized that 

 other factors enter into crop rotations, such as the elimination 

 of associated weeds, various kinds of animal, insect and plant 

 parasites, preparation of the soil by a deep-rooted crop for a 

 shallow-rooted following crop, etc. It has come to be recog- 

 nized that there are natural associations of plants, and natural 

 rotations of vegetation certainly determined by other than plant- 

 food factors. Thus, in the eastern United States, wheat is 

 followed by ragweed naturally, while across the fence cocklebur 

 and wild sunflower come in after the corn, the difference in 

 vegetation being as sharply marked after the removal of the 

 crops as when they still occupied the land. Analyses of the 

 ragweed, for instance, although it is a shallower rooted crop 

 than wheat, show that it takes from the soil as much of the 



1 See in this connection, Further studies on ,the properties of unpro- 

 ductive soils, by B. E. Livingston, Bull. No. 36, Bureau of Soils, Dept. of 

 Agric., 1907, pp. 7-9. 



