THE SOIL AIE 477 



centage of water in the soil, the smaller will be the volume 

 of air, since the amount of pore space determines both the 

 water and the air capacity. A soil with 30 per cent 

 moisture may contain more air than one with a w^ater 

 content of 20 per cent, because of the tendency of mois- 

 ture to move the soil particles farther apart. 



In soils in the field, the average diameter of the cross 

 section of the pore space is the most potent factor in 

 determining the volume of air. Small spaces are likely 

 to hold water, while larger spaces, not retaining water 

 against gravity, are filled with air. 



In a clay soil the volume of air is increased, other 

 things being equal, by the formation of granules, and is 

 decreased by deflocculation or compaction. The volume 

 of air in any soil may be calculated from the following 

 formula : — 



% air space = % pore space — (% H2O X ap. sp. gr.) 



COMPOSITION OF SOIL AIR 



The air of the soil differs from that of the outside 

 atmosphere in that it contains more water vapor, a much 

 larger proportion of carbon dioxide, a correspondingly 

 smaller amount of oxygen, and slightly larger quantities 

 of other gases, including ammonia, methane, hydrogen 

 sulfide, and the like, formed by the decomposition of 

 organic matter. 



395. Analyses of soil air. — The composition of the 

 air of several soils, as determined by Boussingault and 

 Lewy, is quoted by Johnson ^ in the table following : — 



1 Johnson, S. W. How Crops Feed, p. 219. New York, 1891. 



