516 STATE BOARD OP AGRICULTURE. 







ing the night the soil is as a rule warmer than the air above." Other 

 writers, such as Ramann^*, etc., claim in substance the identical belief. 

 But really, is there a rising of vapor or warm moist air from the 

 warm soil below to the cold soil above? And is the source of 

 water of the dew due to this soil vajjor? During the day the soil 

 receives its heat at the upper surface and its temperature 

 rises. The heat is conducted downward and the temperature of the 

 various depths of the soil increases correspondingly. The temperature 

 at the surface continues to rise until a maximum is reached and then 

 commences to decrease. As the temperature rises and moves downward 

 the soil air expands, and as the volume of the pore space remains con- 

 stant, it is expelled into the atmosphere. The pressure of the soil air 

 at the different depths tends to be the same at any one time and equal to 

 the atmospheric pressure, provided the communications are ideal. When 

 the temperature at the surface soil is at maximum it is generally many 

 degrees higher than that of the air above, amounting sometimes to 30° 

 C. In fact the air temperature decreases, in calm and clear weather, 

 with increase in height at the adiabatic rate of approximately 0.9° C. 

 per 300 feet. When the temperature of the surface soil and of the air 

 is highest the atmospheric pressure also tends to be at its minimum, so 

 that the air escapes from the soil with greater facility. After the sur- 

 face soil attains its maximum temperature and then begins to cool, its 

 air contracts, tends to produce a partial vacuum, and consequently 

 draws air from the atmosphere, so its pressure will be in equilibrium 

 with that of the latter. The fall of temperature is also conducted down- 

 ward, and proceeds as a wave, and as it descends it causes a diminution 

 in volume at the corresponding depths, and therefore produces an in- 

 ward flow of air. This cold wave, however, is preceded by the maximum 

 temperature wave which, as it proceeds downward, causes a further ex- 

 pansion of air, which goes to make up for the decreased volume of air 

 caused by the cold wave following immediately after. The difl'erence in 

 temperature, however, of the soil at any depth immediately before and 

 after the maximum temperature wave is reached is very small, as ex- 

 periments at this Station show, and consequently the expansion and ex- 

 pulsion of air caused by the downward march of the maximum tempera- 

 ture wave is not very appreciable. Hence, as the cold wave proceeds down- 

 ward and produces a decrease in volume of the soil air, the air that 

 comes to make up for this decrease, so that an equilibrium of pressure 

 will exist, is mainly from the outside atmosphere. After a certain depth 

 is reached the maximum temperature wave entirely disappears, and there 

 is no more upward expulsion or movement of air. From now on, as the 

 temperature of the soil is further decreased and the volume of its air 

 is diminished correspondingly, the current of flow of air into the soil is 

 entirely from the outside atmosphere. This downward flow of air will 

 continue until the soil temperature begins to rise again and the cycle re- 

 commences. When the minimum temperature of the surface soil is 

 reached, it is as a rule, about the same or slightly higher, than that of 

 the air immediately above. The temperature of the air at about this 

 period increases with the height, in the same manner as the tempera- 

 ture of the soil increases with depth, which is just the opposite from 

 what it is during the day. This increase instead of decrease of a 



"Bodenkunde, p. 382 (1911). 



