52 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



phere, and those without move in. This process, known as 

 diffusion, though a slow process, aids in soil ventilation. 



Air expands when its temperature is raised, and contracts 

 when it cools. These changes also aid in soil ventilation. So 

 do the expansion and contraction resulting from barometric 

 changes. Gusts of wind exert a suctional effect. When water is 

 removed from the soil, air enters in its place. 



