442 THE PRINCIPLES OF SOIL MANAGEMENT 



Diurnal Range of Air and Soil Temperatures 



Degrees Fahr. 



Air 5 feet above ground 14.4 



Soil 1 inch below surface 17.9 



Soil 3 inches below surface 14.8 



Soil 6 inches below surface 9.2 



Soil 9 inches below surface 6.6 



Soil 12 inches below surface 4.3 



Soil 24 inches below surface 0.5 



Soil 36 inches below surface 0.0 



This soil contains about 50 per cent of pore space, in 

 the upper foot of which 40 per cent is normally filled 

 with water during the summer months. This leaves 518 

 cubic inches of air in the upper cubic foot of soil. With 

 an increase in temperature, the air expands 79T m volume 

 for each degree Fahr. The average increase of tempera- 

 ture is, in this case, about 11 Fahr. for the first foot. 

 The air exhaled or inhaled by each cubic foot of soil 

 would then be 



' r = 11.6 cubic inches. 



491 



As this is slightly over 2 per cent of the air contained 

 in the upper foot of soil, and as the movement below 

 that depth is negligible, the change in composition at any 

 one time is not great; but this pumping effect is kept up 

 day after day, although less energetically in the cooler 

 portion of the year. In proportion as poor drainage 

 equalizes the temperature it would prevent this type of 

 circulation. The total effect assisted by diffusion is to 

 aid materially in ventilating the soil. Owing to diffusion 

 of air in the interstitial spaces, the air expelled is dif- 

 ferent in composition from that inhaled. 



