Conditions Influencing Soil Temperatures. 219 



especially if covered with lumps, the large amount of sur- 

 face exposed to the sky and to the air permits the heat of 

 the surface soil to be lost rapidly in warming the air above 

 and the result is the deeper soil remains at a lower tem- 

 perature. So, too, if the soil is loose and open, the dry 

 superficial layer becomes warm and heats the air, while 

 the poor conducting capacity of the open soil prevents the 

 heat from being conveyed deeply below the surface and a 

 lower temperature is the result. 



265. Influence of Surface Tillage on Soil Temperature. 

 When corn rround was cultivated 3 inches deep as com- 

 pared with 1.5, in alternate groups of four rows, the moan 

 temperatures of the soil in the first, second and third feet 

 below the soil stirred was found to be .82 F. warmer in 

 the first foot and .59 F., and .36 F. respectively in the 

 second and third feet on the ground receiving the shallower 

 cultivation. 



266. Influence of Chemical and Physical Changes on Soil 

 Temperature. When heavy dressings of farmyard manure 

 are plowed in, and when heavy crops are turned under for 

 green manure, the fermentation which is set up in these 

 materials results" in a measure of heat which warms the 

 soil in the same way that a manure heap heats when fer- 

 menting. Indeed all of the steps in the formation of ni- 

 trates in the soil result in the evolution of some heat. 



Again, when the surfaces of dry soil grains become mois- 

 tened with water, whether by rain or by capillary move- 

 ments, surface tension in forcing the water to surround 

 the soil grains generates a small amount of heat, which 

 affects, in so far, the soil temperature. 



267. Influence of Eains on Soil Temperature. Heavy 

 rains which fall upon fields and penetrate the soil may ex- 

 ert very marked effects upon its temperature on account of 

 the relatively high specific heat of the water as compared 

 with that of the soil. 



If the atmosphere is warmer than the deeper soil, as 



