260 NATURE AND PROPERTIES OF SOILS 



Not only are the amounts of air expelled larger than the 

 theoretical figures, but the differences rise with the tempera- 

 ture. With a change of 40° C. it is to be expected that the 

 actual gas expelled will exceed the theoretical from 1.2 to 2.7 

 times, depending on the soil and its condition. This apparent 

 discrepancy is due to the expansion of the aqueous vapor in 

 the soil air and to the liberation of absorbed gases with a rise 

 in temperature. 



Diurnal fluctuations in temperature often rise as high as 

 15° C. for the upper six inches of soil in the summer months. 1 

 When it is remembered that monthly and seasonal differences 

 are even greater than the diurnal and that this respiring effect 

 continues day after day, the importance of temperature in 

 relation to air movement cannot be minimized. If a six-inch 

 layer of soil is raised from 5° C. to 20° C. in temperature, 

 about 10 per cent, of its atmosphere will be expelled, pro- 

 viding the actual expansion is twice the theoretical. 



The wide difference in the compositions of soil and atmo- 

 spheric gases give rise to diffusion movements, especially of 

 the oxygen and carbon dioxide. This tendency towards equi- 

 librium is also important in the readjustments within the soil. 

 As oxidation and carbon dioxide production do not occur 

 equally in all parts of the soil, diffussion movements might 

 easily be induced. The readjustments and equalizations be- 

 tween the soil air proper and that absorbed by the soil-mois- 

 ture are probably largely diffusive. Although diffusion phe- 

 nomena are slow, Buckingham 2 considers them quite impor- 

 tant. 



Waves of high or low atmospheric pressure, frequently in- 

 volving a change of 0.5 inch on the mercury gauge, are con- 

 stantly following each other eastward across the continent. 

 Low pressure allows the soil air to expand and issue from 



*See Swezey, G. D., Soil Temperature at Lincoln, Nebraska; Nebr. 

 Agr. Exp. Sta., 16th Ann. Rep., pp. 95-102, 1903. 



2 Buckingham, E., Contributions to Our Knowledge of Aeration of 

 Soils; U. S. Dept. Agr., Bur. Soils, Bui. 25, 1904. 



