CARBON DIOXIDE IN THE SOIL 



159 



face, carbon dioxide occupies a greater portion of the pore spaces 

 in the deeper regions of the soil (see Table 37) . The air of the soil 

 is thus considerably higher in carbon dioxide than the normal 

 atmosphere above the soil. While the atmosphere contains close 

 to 0.03 per cent of the gas, the soil air shows from 0.05 to 3.8 per 

 cent, and seldom as low as the smallest figure. Table 38 gives 

 figures for the composition of the gases in soils under aerobic and 

 anaerobic conditions. 



TABLE 37 



Carbon Dioxide in the Soil Air at Different Distances below the 

 Surface (from Lau) 



Since diffusion of the soil gases to the atmosphere is more rapid 

 near the surface, the relative carbon dioxide concentration will 

 be greater at lower depths. In the deeper layers the movement of 

 the gases is retarded by the longer distance required to be traveled 

 and the smaller number of channels opening to the surface. Soil 

 water depresses the speed of movement to an important extent by 



TABLE 38 



Composition of Soil Gases 



Aerobic (from Russell and 

 Appleyard) 



Anerobic or rice paddy 



soils (from Harrison 



and Aiyer) 



Oxygen 



Carbon dioxide 



Nitrogen 



Methane 



Hydrogen 



18.00 to 21.00 per cent ± 



0.06 to 2.0 per cent ± 



79 per cent ± 











or traces 



1-20 per cent 



10-95 per cent 



15-75 per cent 



0-10 per cent 



