144 SOILS AND FERTILIZERS 



of soil air is desirable, but there can be too much. On the 

 other hand, there may be too little. It may be assumed 

 that when a soil is in a compact condition it has an insuffi- 

 cient supply of air. 



181. Movement of soil air. — The rate at which air moves 

 through a soil depends largely on the size of the pore spaces, 

 rather than on their aggregate volume. Movement of air 

 is necessary to ventilate the soil, just as it is to freshen the 

 air in a house in which many persons live, or a public hall 

 in which people congregate. Among the factors concerned 

 with the movement of soil air are (1) movement of water, 

 (2) diffusion of gases, (3) some minor conditions, like dif- 

 ferences in temperature between atmospheric air and soil 

 air, periodic changes in atmospheric pressure and suction 

 produced by wind. 



182. Movement of water. — The movement of soil air 

 caused by water is probably the most important of any. 

 When rain falls, the surface soil first receives the water, 

 which usually fills all of the spaces between the particles. 

 As the water descends, air is driven from the pore spaces 

 to make room for the water, the air escaping upward as the 

 water goes downward, or else being forced out through the 

 drainage channels below. The movement of air proceeds 

 to the depth of the water table. Fully one-fourth of the 

 air in a soil may be forced out by a normal- change in the 

 moisture content of a soil. As the soil dries out air returns. 



183. Diffusion of gases. — Owing to the difference in com- 

 position between the atmospheric air and soil air, there is a 

 tendency for them to mix, and this process would go on until 

 the two had the same composition, were it not for the fact 

 that gases are continually being formed in the soil and thus 

 prevent the soil from attaining the same composition as the 

 atmospheric air. The process of diffusion is, therefore, con- 

 tinuous. 



