258 NATUEE AND PROPBETIES OF SOILS 



It mtist also follow that the larger the proportion of the 

 interstitial space filled with water, the smaller will be the 

 quantity of air contained. This does not mean that the soil 

 with the higher percentage of water will contain the least air. 

 The percentage pore space, which is determined by the tex- 

 ture, strneturfi, and organic matter is a consideration also. 

 These three factors, together with moisture content, are in- 

 volyed in the following formula for calculating air space: 



% Air Space = % Pore Space — (%Bi,0 X Vol. Wt.) 



If one soil, containing 30 per cent, of water, has a pore 

 space of 50 per cent, and a volume weight of 1.3, its air space 

 would be 11 per cent, of the total soil volume. Another soil 

 with 20 per cent, of moisture, a pore space of 40 per cent. 

 and a volume weight of 1.6 would, on the other hand, con- 

 tain only 8 per cent, of air. The above formula, however, 

 is irremediably inaccurate in two respects. It does not allow 

 for the air dissolved in the soil-moisture nor does it compen- 

 sate for the influence of the gelatinous colloidal material that 

 exists in the interstices especially of a heavy soil. 



135. Movement of soil air. — There seems to be a slow 

 but constant movement of air through the interstitial spaces 

 of a normal soil in an attempt to create a homogeneous com- 

 position within the soil as well as to establish equilibrium with 

 the atmospheric air. The major controls of such movement 

 are (1) moisture and (2) temperature changes. The minor 

 influences are (1) diffusion and (2) fluctuations in atmo- 

 spheric pressure. 



As water, when present in a soil, occupies certain of the 

 interstitial spaces, it decreases the air space when it enters 

 the soil and increases it when it leaves. The downward move- 

 ment of rain-water produces a movement of soil air by forcing 

 it out through the drainage channels 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 



