84 KANSAS UNIVERSITY SCIENCE BULLETIN. 



other possibility which presents itself is the fact that a specific 

 type of by-products may tend to favor certain species of bac- 

 teria, which would cause a disturbance in the equilibrium of 

 the soil flora. Brown 7 has very recently attempted to determine 

 the influence of various crop rotation systems on the nature 

 of the soil bacterial flora. He has shown that a two- or four- 

 year rotation tends to maintain a higher ammonifying- and 

 nitrifying coefficient on the part of the soil than continuous 

 one-cropping. 



Another aspect to this problem of the reduction in crop yields 

 due to continuous cropping is presented by the statements of 

 Bolley, s in which he endeavors to explain the cause of a type 

 of "soil sickness" prevalent in the north-central states of the 

 Mississippi valley. Here repeated one-cropping has been 

 largely practiced with wheat and flax respectively. The result 

 has been a very marked decrease in the crop yields. This de- 

 crease seems hardly explicable on the basis of a rapid reduction 

 in the supply of available plant food, although this is the ex- 

 planation which has invariably been given for this phenomenon. 

 Reference has already been made to de Candolle's explanation, 

 namely that the reduction in crop yield is not due to a starva- 

 tion, but rather to a poisoning occasioned by the accumulation 

 of plant-root excretions. Bolley, however, claims that con- 

 stant one-cropping permits the possibility of an accumulation, 

 not of toxins, but rather of parasitic fungi and spores patho- 

 genic to the crop constantly grown. On this basis, after re- 

 peated one-cropping, the soil becomes more or less extensively 

 contaminated with such disease-producing fungi. Such a con- 

 tamination with disease-producing factors would mean an in- 

 creased percentage of sick and infected plants with a resulting 

 d'minution in crop yield. It is evident that Bolley does not 

 attribute the reduced yield to a decrease in available plant food> 

 and in this respect his explanation harmonizes with the view 

 held by Cameron and Whitney, 9 who are respons : ble for the 

 following statement : 



"Soils are unproductive not because of the lack of the proper 

 nutrients, but because they contain substances actually del- 

 eterious to plant growth." 



7. Brown, Tech. Bulletin No. 6, pt. II, Iowa Agricultural Experiment Station. 



8. Bolley, Science, Oct. 21, 1910. 



9. Cameron and Whitney, Bulletins 22 and 40, Bureau of Soils. 



