SOIL AIR 249 



up by water, is free to move from place to place and is satu- 

 rated or nearly saturated with water-vapor. It is the soil 

 atmosphere most commonly referred to and its composition 

 is set forth in the above tabulation. After this air was drawn 

 off Russell and Appleyard found that still more air could 

 be removed by applying suction. This air at first carried 

 considerable oxygen but by continuing the suction almost pure 

 carbon dioxide was obtained. The amount of gas removed 

 by lowering the pressure varied directly with the moisture 

 content of the soil and consequently it may be considered as 

 air largely absorbed by the moisture of the soil complexes. 



Two types of atmosphere, therefore, exist in the soil. One, 

 the ordinary soil air, is comparatively rich in oxygen. The 

 other, absorbed by the soil moisture, is very low in oxygen 

 but very high in carbon dioxide. Obviously they insensibly 

 merge. The biological significance of these atmospheric types 

 is very important. Their simultaneous presence admits of 

 both aerobic and anaerobic biological activity. For example, 

 rapid nitrate formation might be progressing but no accumu- 

 lation would be evident, due to just as rapid a synthetic activ- 

 ity of the anaerobic forms. 1 



It must not be assumed from the data above quoted that 

 the composition of the soil air is at all constant or that it is 

 approximately the same in every soil. The soil is dynamic 

 in nearly every phase and is nowhere more changeable than 

 in its atmospheric composition. This variability will of course 

 be more marked and more important in the air which occupies 

 the interstitial spaces, although the absorbed air will show 

 some fluctuation. The compositions of the air of several soils, 

 as determined by Boussingault and Lewy 2 are quoted in the 

 following table : 



1 Gainey, P. L., Beal and Apparent Nitrifying Power of Soils; Science, 

 N. S., Vol. 39, pp. 35-37, 1914. 



Doryland, C. J. T., Influence of Energy Material upon the Eelation of 

 Soil Microorganisms to Soluble Plant Food; N. Dak. Agr. Exp. Sta., 

 Bui. 116, pp. 318-399, 1916. 



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



