49 



These chemical and micro-organic effect which 

 follow decrease in soil aeration have been investigated 

 only very superficially and are too little known for 

 detailed analysis. It is manifest, however, that the 

 effects of soil aeration on the plant may be secondary 

 as well as primary and that the fact that bad aeration 

 is injurious and good aeration beneficial, does not 

 necessarily mean anything as to the aerobic or anaerobic 

 character of root respiration. Even the observed good and 

 bad effects of good and bad aeration may be complicated 

 by the effects of the other factors which are changed, 

 purposely or accidentally, at the same time that the 

 aeration is changed. 



It is interesting that the number of bacteria 

 in the soil increases down to a depth of about one half 

 meter, below which it decreases again. 1 Very few bacteria 

 are found below 3 meters in agricultural soils. This 

 distribution of the bacteria may be a response to aera- 

 tion but it is equally possible that it is caused by the 

 supply of organic food, nothing is known concerning the 



1. Fraenkel,- Zeits. fur Hyg. 5: 332 11889); 

 Hilgard,- Soils, pp. 145, 282; Warming,- Oecologjt, p. 

 79 (1909). 



