310 



SOIL PHYSICS AND MANAGEMENT 



The structure of the soil plays some part in the amount of air. 

 This is especially true of fine-grained soils. If granulation exists 

 the space between the granules will be largely occupied by air, even 

 when the soil is well supplied with water. This may increase the 

 amount of air so that it will compare very favorably with that in 

 sand. The amount of organic matter present influences both the 

 water retained and the porosity, but as a general rule it will in- 

 crease the amount of air in soils, since it also increases the granu- 

 lation. The most important factor in determining the amount of 

 air in soil is moisture, which varies from week to week. After a 

 heavy rain air may occupy only a small fraction of the total pore 

 space. With the removal of the water by percolation, evaporation, 

 and by roots the amount of air increases. 



Composition of Soil Air. While the soil air contains sub- 

 stances that are not found to any extent in the air above, yet in 

 general the same elements and constituents are found in it as in the 

 atmosphere. Thus we find the atmosphere composed of oxygen, 

 nitrogen, and carbon dioxide, with a few other elements or com- 

 pounds. In the soil air we find the same elements present, but not 

 in the same proportion. The carbon dioxide is much more abundant 



Composition of Sail Air as Determined by Boussingault and Lewy 1 



Per rent by volume 



Ordinary air (above the surface) 



03 



20.93 



79.04 



