SOIL AIR 261 



the soil, while a high pressure following causes the outside 

 air to enter. An appreciable, but not important, movement 

 of soil air is produced in this way. Gusts of wind, by affect- 

 ing the air pressure, would function in the same way but 

 presumably would influence only the superficial air spaces. 



136. Practical modification of soil air. — The ordinary 

 operations of tillage greatly influence the ventilation of the 

 soil. When a soil is plowed, the bottom of the furrow is ex- 

 posed directly to the air, and, by the separation of adhering 

 particles and aggregates of particles, air is brought into con- 

 tact with portions that previously have been shut off from 

 atmospheric influence. It is partly because of its effect on 

 soil ventilation that plowing is beneficial. The necessity for 

 its practice is obviously 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 listing corn in semi-arid regions, by 

 which the soil is sometimes left unplowed for a number of 

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



Subsoiling, by loosening the subsoil, increases the ventila- 

 tion at the lower depths. Rolling and subsurface packing 

 both diminish the volume and the movement of air. Their 

 essential difference is in their effect on moisture rather than 

 on air. Harrowing and cultivation have the opposite effect, 

 and both may under certain conditions increase the produc- 

 tion of nitrates in the soil by promoting aeration. 



Farm manures, lime, and other amendments that improve 

 the structure of the soil have for that reason a beneficial 

 action on soil aeration. By their effect on the physical con- 

 dition of the soil, they increase its permeability, and by stim- 

 ulating oxidation and carbon dioxide production they induce 

 diffusion. 



Under-drainage, by lowering the water-table and removing 

 the soil-water from the larger capillary spaces, markedly in- 

 fluences the aeration of the soil and thus profoundly modifies 

 the chemical and biological activities therein. There is a 



