478 SOILS: PROPERTIES AND MANAGEMENT 



There are several factors that influence the composi- 

 tion of the soil air, those of greatest importance being 

 the production and escape of carbon dioxide. 



396. Sources of carbon dioxide in soil air. The 

 presence of carbon dioxide in soils is due in small part 

 to infiltration from the atmospheric air, there being a 

 tendency for the carbon dioxide, which is heavier than 

 oxygen and nitrogen, to settle out. It may also have a 

 purely chemical origin. But in much greater measure 

 is the carbon dioxide a product of biological processes 

 that occur in the soil. At one time it was believed that 

 the formation of carbon dioxide in soils was a purely 

 chemical process of oxidation, and possibly a part of the 

 gas is formed in that way. It has already been seen that 

 there is a condensation of gases in the manifold pores 



