484 SOILS: PROPERTIES AND MANAGEMENT 



it leaves. The downward movement of rain water pro- 

 duces a movement of soil air by forcing it out through 

 the drainage channel below, while at the same time a 

 fresh supply of air is drawn in behind the wave of satura- 

 tion as the water passes down from the surface. The 

 movement thus occasioned extends to a depth where 

 the soil becomes permanently saturated with water. 

 Twenty-five per cent of the air in a soil may be driven out 

 by a normal change in the moisture content of the soil. 



402. Changes in atmospheric pressure. — Waves of 

 high or of low atmospheric pressure, frequently involving 

 a change of 0.5 inch on the mercury gauge, cross the con- 

 tinent alternately every few days. The presence of a 

 low pressure allows the soil air to expand and issue from 

 the soil, while a high pressure following causes the out- 

 side air to enter in order to equalize the pressure. An 

 appreciable, but not important, movement of soil air is 

 produced in this way. 



The size of the interstitial spaces is more potent than 

 their volume in effecting soil ventilation by this and the 

 following methods. 



403. Changes of temperature in atmosphere or in soil. — 

 A movement of soil air may be induced by a change of 

 temperature in the atmosphere or in the soil itself. Changes 

 in atmospheric temperature act in the same way as do 

 changes in atmospheric pressure; in fact, it is the effect 

 of temperature on air pressure that causes the movement. 

 Like the movement due to atmospheric pressure, it is 

 not great ; but where the soil immediately at the surface 

 of the ground' attains a temperature of 120° F. at midday, 

 as is the case in the Corn Belt, the movement must be 

 appreciable. 



The diurnal change in soil temperature decreases 



