Processes of Soil Ventilation. 207 



PROCESSES OF SOIL VENTILATION. 



The interchange of gases between the soil and atmos- 

 phere is brought about in several ways and by different 

 agencies. Among these are (1) the slow process of diffu- 

 sion described in (5) and (14). (2) The expansion and 

 contraction of soil-air due to changes in temperature. (3) 

 The expansion and compression of the air due to changes 

 in barometric pressure. (4) The suctional effect of the 

 wind, especially when it is gusty. (5) The air absorbed 

 by rainwater is carried into the soil when percolation takes 

 place. (6) When water drains away from a soil or is 

 carried upward and out by capillarity or root action it 

 acts by suction to draw into the soil a volume of air equal 

 to that of the water which flows out. 



244. Ventilation of Soil by Diffusion. The exchange of 

 air between that in the soil and the atmosphere above by 

 diffusion is a very slow process but, because it is all the 

 time taking place, the total exchange during the growing 

 season is considerable. The more open the texture of the 

 soil is and the higher the soil temperature the more rap- 

 idly will the interchange by this process take place. 



245. Soil Ventilation Due to Changes in Soil Tempera- 

 ture. When the temperature of air is changed its volume 

 is also altered and in the ratio of TTT for each degree 

 F. or *$r for each degree C. ; so that if 491 cubic feet 



of soil-air were to have its temperature changed 1 F. this 

 would result in one cubic foot of air being forced cut of 

 the soil, if the temperature was raised, and a like amount 

 would enter if the temperature were to fall the same 

 amount. 



The temperature of the surface three inches of soil often 

 changes as much as 16 to 20 F. and that at 18 inches 

 deep as much as 1.5 F. A soil like the surface foot in 

 (133), containing 18 per cent, of water, would enclose 



