THE SOIL AIR 483 



MOVEMENT OF SOIL AIR 



There is a constant movement of the air in the inter- 

 stitial spaces of the soil and an exchange of gases between 

 the soil atmosphere and the outside atmosphere, as well 

 as a more general, but probably less effective, movement 

 of the air out of or into the soil, as the controlling condi- 

 tions may determine. The movement may be produced 

 by any one or more of the following phenomena : (1) 

 diffusion of gases ; (2) movement of water ; (3) changes 

 in atmospheric pressure; (4) changes of temperature in 

 atmosphere or in soil ; (5) suction produced by wind. 



400. Diffusion of gases. — The wide difference in the 

 composition of soil and atmospheric air gives rise to a 

 movement of gases due to a tendency for the external 

 and the internal gases to come into equilibrium. Accord- 

 ing to Buckingham, 1 the interchange of atmospheric and 

 soil air is due in large measure to diffusion. 



The rate of movement of the soil air due to diffusion 

 is dependent on the aggregate volume of the interstitial 

 spaces, not on their average size. Thus, it is the porosity 

 of the soil that influences most largely the diffusion of 

 the air from it. Consequently the size of the particles 

 is not a factor, but good tilth permits diffusion to take 

 place more rapidly than does a compact condition of soil, 

 as the volume of the pore space is thereby increased. 

 Compacting the soil in any way, as by rolling or trampling, 

 has the opposite effect. 



401. Movement of water. — As water, when present 

 in a soil, fills certain of the interstitial spaces, it decreases 

 the air space when it enters the soil and increases it when 



1 Buckingham, E. Contributions to Our Knowledge of 

 the Aeration of Soils. U. S. D. A., Bur. Soils, Bui. 25. 1904. 



