440 THE PRINCIPLES OF SOIL MANAGEMENT 



to Buckingham, 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 upon the aggregate volume of the interstitial 

 spaces, and not upon their average size. Thus it is the 

 porosity of the soil that influences most largely the dif- 

 fusion of the air from it, and consequently the size of the 

 particles is not a factor, but good tilth permits diffusion 

 to take place more rapidly than does a compact condi- 

 tion 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. 



306. Movement of water. As water, when present 

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

 decreases the air space when it enters the soil and 

 increases it when it leaves. The downward movement 

 of rain-water produces a movement of soil air by forc- 

 ing it out through the drainage channel below, while at 

 the same time a fresh supply of air is drawn in behind 

 the wave of saturation, 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. 



307. Changes in atmospheric pressure. Waves of 

 high or low atmospheric pressure, frequently involving 

 a change of .5 inches on the mercury gage, cross the 

 continent 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- 



