FACTORS AFFECTING SOIL AERATION 441 



side air to enter in order to equalize the pressure. An 

 appreciable, but not important movement of soil air is 

 produced in this way. 



The size of the interstitial spaces is more potent than 

 their volume in effecting soil ventilation by this and the 

 following methods. 



308. Changes in temperature. A movement of soil 

 air may be induced by a change of temperature in the 

 atmosphere or in that of the soil itself. Changes in atmos- 



Fig. 118. The hillside plow. The hinged share and moldboard permit con- 

 tinuous plowing on one side of the land. 



pheric temperature act in the same way as do changes 

 in atmospheric pressure; in fact, it is the effect of tem- 

 perature upon air pressure that causes the movement. 

 Like the movement due to atmospheric pressure, it is 

 not great; but where the soil immediately at the sur- 

 face of the ground attains a temperature of 120 Fahr. 

 at mid-day, as occurs in the corn-belt, the movement 

 must be appreciable. 



The diurnal change in soil temperature decreases 

 rapidly from the surface downward, due to the absorp- 

 tion and slow conduction of heat. (See page 455.) At 

 the Nebraska Experiment Station, the average diurnal 

 range for the month of August, 1891, was as follows: 



