Ways of Influencing Soil Ventilation. . 211 



its action upon the lime, to the flocculation of the minuter 

 soil particles and thus to a more extensive granulation oi 

 the whole subsoil, which in turn extends the soil ventilation 

 still more widely. 



But all of these effects upon the soil are only the means 

 which permit the underdrains to render their greatest serv- 

 ice in permitting a strong and extensive movement of air 

 into and from the soil ; for once the soil is opened up in this 

 way, the air, through the action of the wind, changes in 

 barometric pressure and changes in soil temperature, read- 

 ily enters the soil, not only through the surface above but 

 throughout the whole length of the underdrains. 



When it is seen that changes in soil temperature and in 

 atmospheric pressure make such marked changes in the 

 flow of water from springs and from tile drains as are 

 shown in (337) and (338) it becomes clear that the move- 

 ments of soil-air into and out of tile drains must be even 

 more marked than the movements of ground water. 



252. Influence of Vegetation on Soil Ventilation. In the 

 case of such crops as clover, which send long and somewhat 

 fleshy roots down deeply into the subsoil, there are very 

 many and important passageways opened up alter the roots 

 decay, which greatly facilitate the deeper and more rapid 

 change of soil-air, and, as has been pointed out, the re- 

 moval of water by the living roots must also draw into the 

 soil a volume of air equal to the amount of water used, 

 except in so far as this is made good by the rise of capil- 

 lary water from below. 



