THE SOIL AIR 487 



While a certain movement of air through the soil is 

 desirable, and indeed necessary, for the reasons already 

 stated a very considerable movement is injurious unless 

 there is an abundant rainfall. The effect of air move- 

 ment through the soil is to remove soil moisture. In a 

 region of light rainfall and low atmospheric humidity, this 

 may be disastrous if the soil is not kept compact by care- 

 ful tillage. On the other hand, in a humid region and 

 in clay soil there is likely to be too small a supply of oxygen 

 for the use of crops and lower plant life unless the soil 

 is well stirred. 



405. Tillage. ^ — The ordinary operations of tillage 

 greatly influence the ventilation of the soil. When a soil 

 is plowed, the soil at the bottom of the furrow is exposed 

 directly to the air at the surface, and, by the separation 

 of adhering particles and aggregates of particles, air 

 is brought into contact with particles that may previously 

 have been completely shut off from air. It is partly 

 because of its effect on soil ventilation that plowing is 

 beneficial, and the necessity for its practice is greater 

 in a humid region and on a heavy soil than in a region 

 of light rainfall and on a light soil. The practice of list- 

 ing com, by which the soil is sometimes left unplowed 

 for a number of years, although in semiarid regions pro- 

 ductive of crops of sufKcient yield to make them profitable, 

 would fail utterly on the heavy soils of a humid region. 



Subsoiling, by loosening the subsoil, increases thb 

 ventilation to a greater depth. Rolling and subsurface 

 packing both diminish the volume and the movement of 

 air. Their essential difference is in their effect on mois- 

 ture rather than on air. Harrowing and cultivation have 

 the opposite effect, and both increase the production of 

 nitrates in the soil by promoting aeration. 



