MODIFICATION OF SOIL AERATION 443 



309. Suction produced by wind. The movement of 

 wind, being almost always in gusts, alternately increases 

 and decreases the atmospheric pressure at the surface 

 of the soil. There is a tendency, therefore, for the soil 

 air to escape and for atmospheric air to penetrate the 

 soil with each change in pressure. The effect presumably 

 influences only the superficial air spaces, but it must be 

 very frequent in its action. No measurements have 

 been made and no definite estimate of its effect can be 

 arrived at. 



V. METHODS FOR MODIFYING THE VOLUME AND 

 MOVEMENT OF SOIL AIR 



The conditions that affect the ventilation of soils are: 

 (1) The volume and size of the interstitial spaces. (2) 

 The moisture content. (3) The daily and annual range 

 in temperature. 



Although the size of the interstitial spaces does not 

 appear to influence greatly the diffusion of gases from 

 a soil, it has a marked effect upon certain of the other 

 processes by which air enters and leaves the soil. A 

 sandy soil, a soil in good tilth, and, particularly, a soil 

 composed of clods, permit of more rapid movement of 

 air than does a compact soil. 



While a certain movement of air through the soil is 

 desirable, and indeed necessary, for the reasons already 

 stated, a very large movement is injurious unless there 

 is an abundant rainfall. The effect of air movement 

 through the soil is to remove soil moisture. In a region 

 of small rainfall and low atmospheric humidity, this 



