CHAPTER XXII 

 THE SOIL AIR 



The air of the soil is merely a continuation of the 

 atmospheric air into the interstitial spaces of the soil, 

 when these are not filled with water. As it is more or 

 less inclosed by the soil, movement does not take place 

 so readily as it does above the surface of the ground and 

 hence the soil air is more greatly influenced by its sur- 

 roundings than is atmospheric air. This leads to impor- 

 tant differences in composition between the atmospheric 

 air and soil air, the composition of the latter depending 

 on a variety of conditions in which physical, chemical, 

 and biological properties play a part. 



FACTORS THAT DETERMINE VOLUME 



The amount of air that soils contain varies with their 

 properties, and in any one soil the air content varies with 

 certain changes to which the soil is subject from time to 

 time. The factors that influence the volume of air in 

 soils are: (1) texture; (2) structure; (3) organic matter ; 

 (4) moisture content. 



391. Texture. — The size of the soil particles affects 

 the air capacity of the soil in exactly the same way 

 as it does the pore space, since in dry soil they are 

 identical. A fine-textured soil in a dry condition would 

 therefore contain as large a volume of air as would a 



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