EXPERIMENT STATION BULLETINS. 



553 



soil aeration must be considered from this standpoint also. Hence, the 

 temperature ranges employed in this investigation are within the limits 

 of the diurnal and seasonal changes that actually occur in the soil. 



At this point the reader may appear to be noticing certain discrepan- 

 cies in the conclusions of the present research in comparison with those 

 on the "Movement of Moisture from Moist and Warm Column of Soil to 

 Dry and Cold Column of Soil with an Air Space between the two Col- 

 umns." In this latter research it was concluded that the amount of 

 water moved through vapor, from a point of high to a point of low tem- 

 perature, and the amount of water lost from the soil through diffusion of 

 water vapor, are practically negligible. In the present research, it has 

 been seen that temperature causes the expulsion of enormous amounts 

 of gas from the soil, and that this abnormal increase in volume of gas 

 expelled is due to aqueous vapor. The reader may now ask. How are 

 these facts reconciled, If rise of temperature causes the expulsion of 

 air from the soil which contains moisture, then the amount of water lost 

 from the soil through vapor alone must be very large. The answer to 

 these questions will be found in the following table which contains the 

 amount of water vapor required to saturate a given quantitj^ of air at 

 different temperatures. The amount of water vapor which saturates a 

 given quantity of air is usually expressed in grains per cubic foot or 

 grams per cubic meter, or it may be expressed in terms of the pressure 

 which it exerts in millimeters or inches. 



TABLE 14.— AMOUNT OF WATER VAPOR REQUIRED TO SATURATE A GIVEN QUANTITY 



OF AIR AT DIFFERENT TEMPERATURES. 



Considering the figures of the Centigrade scale it will be seen that the 

 amount of water contained in a cubic meter (1000 liters) of saturated 

 air at 20° C. is only 17.15 grams. That means that if 1000 liters of 

 saturated air are expelled from the soil at the temperature of 20° C. 

 only 17.15 grams of water will be lost from the soil. In table 10, it is 

 shown that by raising the temperature of one-half cubic foot of soil from 

 0° to 50° C. only about two liters of air will be expelled from the mineral 

 soils and about three liters from the peat. On the other hand, the air 

 expelled from the soil as the temperature rises, cannot be said that it is 

 always saturated. Hence, the amount of water that may be lost from the 

 soil through expulsion of soil air incident to soil ventilation, is in- 

 significant. 



Before closing this section, one more point should be considered; 



