THE KANSAS UNIVERSITY 

 SCIENCE BULLETIN. 



Vol. IX, No. 7.J DECEMBER, 1914. [vl H xi\ s To ES 7 



A Contribution to the Subject of the 



Factors Concerned in Soil 



Productivity. 



CONRAD HOFFMANN, 



Secretary of the University of Kansas Y. M. C. A.* 



Plates XXIII to XXVII. 



THE question as to the presence or absence in certain soils 

 of toxic substances which exert a detrimental effect upon 

 the growing" crops is a matter of considerable importance. 

 Ever since the time of de Candolle 1 there has existed a more 

 or less extensive belief that crops excrete substances from their 

 roots which are of a toxic nature. He it was who first at- 

 tempted to explain certain phenomena associated with systems 

 of crop rotation on the basis of plant-root excretions. He 

 asserted that plants excrete from their roots substances that 

 remain in the soil and injure other plants of the same species 

 but do not necessarily affect plants of other species. In a crop 

 rotation these toxic substances are supposedly destroyed dur- 

 ing the period the other species of plants are grown, and so are 

 removed before the original crop is again grown. In case, 

 however, the same crop is continuously grown, these sub- 

 stances not only persist but accumulate, and sooner or later 

 exert a detrimental effect with a resulting diminution in crop 

 yields. In this way de Candolle thus early endeavored to ac- 



Received for publication October 21, 1914. 



* Experimental work for this article was done in the laboratories of the Department of 

 Agricultural Bacteriology of the College of Agriculture, University of Wisconsin, Madison, 

 Wis. 



1. De Candolle, Physiologie Vegetale, 1832, T. Ill, p. 1474. 



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