446 THE PRINCIPLES OF SOIL MANAGEMENT 



profitable ; would fail utterly on the heavy soils of a 

 humid region. 



Subsoiling by loosening the subsoil increases the 

 ventilation to a greater depth. Rolling and sub-surface 

 packing both diminish the volume and movement of air. 

 Their essential difference is in their effect upon moisture 

 rather than upon air. (See page 111.) Harrowing and 

 cultivation have the opposite effect, and both increase 

 the production of nitrates in the soil by promoting 

 aeration. The tillage which is most beneficial is that 

 which increases the porosity of the soil, and not the size 

 of the interstitial spaces. 



311. Manures. Farm manures, lime and those 

 amendments that improve the structure of the soil, have 

 to the same degree a beneficial action upon soil aeration. 

 By their effect upon the physical condition of the soil, 

 they increase its permeability, and by their action in con- 

 tributing to the production of carbon dioxid they stimu- 

 late diffusion. 



It is chiefly through its effect in increasing the volume 

 of air space in soils that farm manure is injurious in light 

 soils of the semi-arid region. It may thus be injurious 

 as well as beneficial, if used under certain conditions. 



312. Under drainage. By lowering the water table, 

 unclerdrainage by means of tiles removes from the soil 

 the water from all but the small capillary spaces, and 

 leaves free to the air the remainder of the interstitial 

 spaces. There is also a very considerable movement 

 of air through the drains, and a movement of air upward 

 from the drains to the surface of the soil, which, serves 

 to aerate, to some extent, this intervening layer. The 



