68 CHEMISTRY OF FARM PRACTICE 



sure within the cup, which will exert pressure upon the 

 water in the connecting bottle, forcing it in a spray from 

 the tube. Should the bell jar with its hydrogen atmosphere 

 now be removed, the hydrogen which now is within the 

 cup will diffuse rapidly into the outside air and the de- 

 creased pressure within the cup will be indicated by bubbles 

 of air rising from the tube through the water in the bottle. 



Thorough tillage enlarges the pore space and aids dif- 

 fusion; packing a soil decreases the pore space and the 

 diffusion. When rain falls, the water fills much of the 

 pore space, excluding a certain volume of air; but, as the 

 water sinks into the soil, the air is forced after it, because 

 of the pressure of the atmosphere above, filling the space 

 made vacant by the sinking of the water. The volume 

 of a gas is directly proportional to the temperature and in- 

 versely proportional to the pressure. The warmer the tem- 

 perature, the greater the volume of any gas, and the greater 

 the pressure, the smaller will be the volume occupied by 

 a given gas. Under climatic conditions, there are con- 

 stant changes of both temperature and pressure, which 

 bring about movement of the soil atmosphere. 



57. Means of Producing a Change of Soil Air. The 

 means at our disposal to produce change of soil air are 

 tillage, underdrainage, rotation, manures, and lime. 



Thorough tillage induces more exchange of air between 

 the atmosphere above the surface and the air beneath the 

 surface. Underdrainage removes superfluous water and in- 

 creases the pore space that is filled by air, thus allowing 

 a freer circulation within the soil. Irrigation induces change 

 in soil air in the same manner that rain induces change. 

 The influx of water to a large extent excludes the air from 

 the soil, and, as the water sinks into the soil, the pore 

 space is refilled with air. Rotation of crops aids in the 

 proper aeration of a soil, because the root systems of the 

 different crops grown in the rotation are confined to dif- 

 ferent soil strata, and, as old roots decay through the 



