208 Physics of the Soil. 



about 5.3 acre-inches of air in the surface 1.5 feet and, 

 with a diurnal change of 16.4 F. in the upper 3 inches 

 and 1.5 F. at a depth of 18 inches, the amount of soil-air 

 which would be forced out and again taken in each 24 

 hours would be about 14 cubic inches for each square foot 

 of surface. So that the soil ventilation due to diurnal 

 changes in soil temperature will range from up to pos- 

 sibly 20 cu. in. per square foot. 



246. Influences of Changes in Barometric Pressure on Soil 

 Ventilation. Any change which may occur in the pressure 

 of the air above the soil is followed by a change in the 

 volume of the soil-air, causing an escape from the soil, if 

 the pressure above falls, and the entrance of an extra sup- 

 ply whenever the pressure is increased. 



With soil like that in (133), having 18 per cent, of water 

 in the first foot, 20 per cent, in the second and 15 per 

 cent, in the third and fourth feet, there would be 7.88 

 inches in depth of soil-air contained in the four feet and 

 every change in atmospheric pressure amounting to .1 incli 

 would cause the escape or entrance of 3. 78 cubic inches 

 for each square foot of surface and 18.9 cubic inches for 

 each change in pressure of .5 inches of barometer. 



It is common in the United States for waves of high 

 and low pressure to pass a given locality about twice each 

 week, and the differences in pressure between high and low 

 barometer are generally not far from .5 inch, so that the 

 results stated above give a fair measure of this influence 

 in soil ventilation. 



247. Wind Suction and Soil Ventilation. It is seldom 

 true that the wind blowing across a field has a uniform 

 velocity, the general tendency being for it to blow in gusts. 

 This unsteady action tends at times to increase the pres- 

 sure on the soil-air and at other times to decrease that 

 pressure and, as a result, there is a nearly constant ten- 

 dency for air to leave or enter the soil on this account, 

 and it is possible that this factor in soil ventilation may 



