CHAPTER VII 

 AIR IN SOILS 



52. Composition of the Atmosphere. The atmosphere 

 consists mainly of a mixture of the two gases, nitrogen 

 and oxygen, and contains in addition argon, variable quan- 

 tities of aqueous vapor, and very small amounts of carbon 

 dioxide, ammonia, hydrogen, and ozone. Under certain 

 conditions other gases, certain salts, finely divided soil 

 particles, and small particles of animal and vegetable matter 

 may occur as incidental ingredients. Dry air contains about 

 75| per cent by weight of nitrogen, and about 23 per cent 

 by weight of oxygen, the other elements and compounds 

 mentioned being present in very small quantities in mois- 

 ture-free air. Carbon dioxide is present on an average 

 in the proportions of 4 parts of carbon dioxide to 10,000 

 parts of air. This seems insignificant, but the tremendous 

 weight of the atmosphere can be realized when we con- 

 sider that this minute proportion is equivalent to 28 tons 

 of carbon dioxide in the atmosphere over one acre of land. 

 The atmosphere is continuously moving, so that the air 

 over an acre of land is renewed many times during the 

 course of a day, thus tending to keep the air, although a 

 mixture, approximately of definite composition. Growing 

 crops rapidly use up carbon dioxide in the process of build- 

 ing up the plant structures, all of which are largely car- 

 bonaceous. The ratio between oxygen and carbon dioxide 

 is kept constant, the amount of oxygen used up by com- 

 bustion and life processes being restored by the decomposi- 

 tion of carbon dioxide by the chlorophyl of the growing 

 plant and the discharge into the air of the oxygen thus 

 produced. 



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