486 SOILS: PROPERTIES AND MANAGEMENT 



one time is not great ; but this pumping effect is kept up 

 day after day, although less energetically in the cooler 

 seasons of the year. In proportion as poor drainage 

 equalizes the temperature it would prevent this type of 

 circulation. The total effect, assisted by diffusion, is to 

 aid materially in ventilating the soil. Owing to diffusion 

 of air in the interstitial spaces, the air expelled is different 

 in composition from that inhaled. 



404. 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 

 stated. 



METHODS FOR MODIFYING THE VOLUME AND THE MOVE- 

 MENT OF SOIL AIR 



The conditions that influence the 'ventilation of soils 

 are: (1) volume and size of the interstitial spaces; (2) 

 moisture content; (3) daily and annual range in tem- 

 perature. 



Although the size of the interstitial spaces does not 

 appear to greatly influence the diffusion of gases from 

 a soil, it has a marked effect on certain of the other pro- 

 cesses 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. 



