68 



of soil or air, or of atmospheric pressure, as, for 

 instance, by wind eddies, may be locally or momen- 

 tarily important but can scarcely have general signifi- 

 cance. 



Next to diffusion, and perhaps even more im- 

 portant than diffusion, as a cause of soil ventilation 

 are the air movements due to movements in the soil water. 

 It is obvious that the flooding and draining of a soil 

 will first displace the soil air and then renew it 

 with fresh air drawn from the atmosphere. This same 

 action occv.re on a lesser scale every time the soil is 

 wetted or dried. She nature of the capillary water 

 system of the soil has already been mentioned. 1 By 

 virtue of the surface-tension forces on the innumerable 

 curved surfaces of this water system, the water films 

 are able to thicken or to become thinner as water is 

 supplied or withdrawn, and without any general distrac- 

 tion of the water system as a whole. In this way the 

 capillary system serves as an important storehouse of 

 water in soils which are below saturation. As the water 

 films become thinner or thicker in response to changes 

 in the amount of water, it is obvious that air will be 

 pulled into the soil or forced out of it. Every shower 

 thickens the capillary water system and forces air out 



1. Pages if** and (pj above. 



