434 THE PRINCIPLES OF SOIL MANAGEMENT 



centage of water in the soil the smaller the volume of 

 air, as 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 water 

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

 ture to move the soil particles further 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 the larger ones, 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 

 decreased by deflocculation or compaction. 



II. COMPOSITION OF SOIL AIR 



The air of the soil differs from that of the outside 

 atmosphere in containing more water vapor, a much 

 larger proportion of carbon dioxid, a correspondingly 

 smaller amount of oxygen, and slightly larger quantities 

 of other gases, including ammonia, methane, hydrogen 

 sulphid, etc., formed by the decomposition of organic 

 matter. 



299. 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 on the follow- 

 ing page. 



There are several factors influencing the composition 

 of the soil air, those of greatest importance being the 

 production and the escape of carbon dioxid, while of 



