67 



also, by the fact that the nitrogen content of the 

 soil air remains approximately constant . 



Although the investigations of Buckingham are 

 satisfactorily final with regard to the effects of dif- 

 fusion and of atmospheric pressure on the composition of 

 the soil air it would be a mistake to assume that the 

 rates of gas exchange in the soil are limited by the val- 

 ues which he obtains. There are at least two other factors 

 of importance in assisting the ventilation of the soil, 

 namely, temperature changes and movements of the soil 

 water. It is probable that temperature changes are 

 less important than are usually beiieved. Changes in 

 air temperature have no effect except as they alter 

 atmospheric pressure, and such effects have been shown 

 by Buckingham to be slight. Differences in the tempera- 

 ture of soil and of air may set up convectional circu- 

 lations if the soil is the warmer, but it is improbable 

 that such circulations would extend more than a few 

 inches into the soil. Lore general circulations are con- 

 ceivable where* a hillside soil has a temperature substant- 

 ially different from the temperature of the air. In such 

 cases convectional circulations through the soil up 

 or down the slope might occur. It is unlikely, however, 

 that the conditions necessary for such circulation ate 

 often realized, local extreme variations of temperature 



1. dee page , above. 



