THE SOIL AIR 485 



rapidly from the surface downward, due to the absorp- 

 tion and slow conduction of heat (see par. 227). At 

 the Nebraska Experiment Station 1 the average diurnal 

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



DIURNAL RANGE OF AIR AND SOIL TEMPERATURES 



Degrees Fahrenheit 



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 fifty per cent of pore space, in 

 the upper foot of which forty 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 -j-^j- in volume 

 for each degree Fahrenheit. The average increase of 

 temperature is, in this case, about 11 degrees Fahrenheit 

 for the first foot. The air exhaled or inhaled by each 

 cubic foot of soil would then be 



518 X 11 i 



= 11.6 cubic inches 

 491 



As this is slightly over two 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 



1 Swezey, G. D. Soil Temperatures at Lincoln, Nebraska. 

 Neb. Agr. Exp. Sta., 10th Ann. Kept., pp. 95-102. 1903. 



