SOIL AIR 



259 



movement thus occasioned extends to a depth where the soil 

 becomes permanently saturated with water. Twenty-five per 

 cent, of the air in a soil may be driven out by normal change 

 in moisture content. Capillary movement, whether it be pro- 

 duced by evaporation, plant action or other normal forces, 

 likewise produces movement of the soil air. In fact, every 

 readjustment of soil-moisture, however slight, will produce 

 a corresponding adjustment of the air films. 



It is generally considered that the effect of normal tempera- 

 ture change on the contraction or expansion of the soil air is 

 so slight as to produce but little movement. Ramann says, 1 

 " Since the coefficient of expansion of gas is only 1/273 to a 

 degree Centigrade and since the temperature fluctuations to 

 the depths of from four to eight inches are small, the diurnal 

 exchange of gas is consequently slight. ' ' Bouyoucos, 2 by rais- 

 ing the temperature of both dry and moist soil held in a 

 properly controlled apparatus, was able to measure the 

 amount of air actually expelled. He found in every case that 

 the gases driven off markedly exceeded the theoretical 

 amounts. 



Table LVI 



EFFECT OF TEMPERATURE ON THE AMOUNT OF AIR EXPELLED FROM 



MOIST SOILS 



1 Eamann, E., BodenTcunde, Seite 386; Berlin, 1905. 



2 Bouyoucos, G. J., Effect of Temperature on Some of the Most 

 Important Physical Processes in Soils; Mich. Agr. Exp. Sta., Tech. Bui. 

 22, pp. 50-62, 1915. 



