82 KANSAS UNIVERSITY SCIENCE BULLETIN. 



count for the better crop yields secured by rotation than by 

 continuous one-cropping. 



This explanation of de Candolle's has been largely retained 

 up to the present time. As evidence of this one finds the follow- 

 ing statement in a recent bulletin of the Bureau of Soils : 2 



"We must regard the excreta of growing roots as one of the 

 main causes of the low crop yields obtained in improper crop 

 rotation." 



In other words, the toxicity which apparently exists in cer- 

 tain soils is thus attributed in large measure to the root ex- 

 cretions of plants. The work of Czapek and others, however, 

 seems to indicate but little excretive activity on the part of 

 plant roots. One is thus forced to look elsewhere for an ex- 

 planation as to the source of these so-called toxins, if such 

 exist in the soil. 



A more frequent explanation of the diminishing crop yields 

 is that the successive crops remove plant food, so that the 

 subsequent crops have less available food and hence are ob- 

 viously unable to produce such large yields. In connection 

 with this latter explanation it is well to mention the investi- 

 gations of Cameron, Whitney, Schreiner, Livingston 3 and 

 others. Their work has been largely responsible for focusing 

 research on the subject of soil toxicity as a cause of reduced 

 soil productivity. According to them the reduced yields are 

 not due to a lack of plant food, but rather to the presence of 

 certain harmful substances in the soil solution. They believe 

 that the concentration of the soil solution is practically the 

 same in all soils. 



Evidence both disproving and substantiating the statements 

 of the above investigators relative to the presence of such soil 

 toxins has been secured. It is an established fact that a del- 

 eterious effect upon crops is frequently exerted by an excess 

 of certain mineral compounds. Of the latter, salts of mag- 

 nesia, iron pyrites and sodium chloride are most frequently 

 associated with such harmful effects exerted by soils on grow- 

 ing crops. The presence of excessive amounts of nitrates is 

 likewise frequently responsible for such inhibitive action. 

 This fact is well demonstrated by the work of Headden and 

 Sackett. 4 They have shown that in certain localities of Colo- 



2. Bulletin 40, Bureau of Soils, U. S. Dept. Agriculture, p. 40. 



3. Bulletins 22, 23, 28, 30, 36, 40, 47, 80, Bureau of Soils, Dept. Agriculture. 



4. Bulletins 155 and 160, Colorado Agricultural Experiment Station. 



