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 ont by normal change 

 in moistnre 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,^ 

 ''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,^ 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 L VI 



EFFECT OF TEMPERATURE ON THE AMOUNT OF AIR EXPELLEI) FROM 



MOIST SOILS 



Soil 



Per 



Cent 



Mois- 



TXJKE 



Peb 



Cent 



Po- 



EosiTy 



Odbic Centimetbbs op Air Expelled 

 Peom ONE-HAT.r Cubic Foot op Soil 



Thbo- 



eetical 



In- 



crease 





0-10°C 



10°-20°C 



20°-30°C 



SO'^O^O 



FOR 



Bach 

 10°C 



Sandy loam 

 Silt loam. 



Clay 



Peat 



11.0 

 18.6 

 25.3 

 92.0 



48.2 

 47.0 

 50.3 

 38.6 



289 

 326 

 363 

 466 



354 

 335 

 382 

 512 



382 



428 

 428 

 559 



419 

 465 

 503 

 657 



250 

 244 

 261 

 200 



^Ramann, E., BodenTcunde, Seite 386; Berlin, 1905. 



^Bouyoueos, G. J., Hffect of Temperature on Some of the Most 

 Important Physical Processes in Soils; Mich. Agr. Exp. Sta., Teeli, BuL 

 22, pp. 50-62, 1915. 



