262 NATURE AND PROPERTIES OF SOILS 



very considerable movement of air in and out of tile drains, 

 which cannot fail to influence the aeration of the soil above. 

 The influence of irrigation on the soil is much like that of 

 rainfall. The alternate filling and emptying of the interstitial 

 spaces with water causes a very considerable change of air. 



The roots of plants left in a soil after the crop has been 

 harvested decay and leave channels in the soil through which 

 air penetrates. Below the furrow slice, where the soil is not 

 stirred and where it is usually more dense than at the surface, 

 this affords an important means of aeration. The absorption 

 of moisture from the soil by roots also causes the air to pene- 

 trate, in order to replace the water withdrawn. 



137. Resume. — The air of the soil differs from the atmos- 

 pheric air in being relatively lower in oxygen and compara- 

 tively very much higher in carbon dioxide. It is generally 

 saturated with water- vapor. The percentage of nitrogen and 

 other gases is about the same as in the atmosphere. The 

 major portion of the soil atmosphere exists in the larger inter- 

 stices. Its movement in most cases is due to moisture and 

 temperature changes, although diffusion and fluctuations in 

 barometric pressure are of some importance. A minor portion 

 of the soil air is dissolved in the soil-water, the absorptive 

 influences of the soil complexes probably playing a part also. 

 Carbon dioxide is the predominating gas in the minor por- 

 tion, which maintains an equilibrium with the more active 

 soil air largely by diffusion. 



While the amount of air in the soil varies with the texture, 

 structure, and organic matter, the moisture content seems to 

 be the dominant factor with volume as well as with movement. 

 Although plowing, tillage, and manuring profoundly influence 

 the soil air and its relationships to normal chemical and bio- 

 logical reactions, natural forces and processes, once the crop 

 is on the soil, seem to control aeration. 



