312 SOILS: PROPERTIES AND MANAGEMENT 



At first glance it appears peculiar that the heat move- 

 ment through a soil, the mineral constituents of which 

 possess a conductivity coefficient of about .010(50, should 

 be raised by the addition of a liquid possessing a value of 

 K of about .00149, a conductivity about one-seventh of 

 the soil minerals. The explanation of this as given by 

 Patten is a lowering of the transfer resistance. He has 

 calculated that heat will pass from soil to water approxi- 

 mately one hundred and fifty times more easily than 

 from soil to air. This being true, it is evident that as 

 the water is increased in any soil and the air decreased, 

 the conductivity coefficient increases. It must be kept 

 in mind, however, that as the moisture increases, the 

 total amount of heat necessary to raise this soil to a given 

 temperature must also be increased. The necessity for 

 the maintenance of a medium moisture content in any 

 soil becomes apparent, although the conductivity may 

 not thereby be at its maximum. The curves in question 

 show that not only is there a change of volume weight, 

 but also there is a decrease in diffusivity with high water 

 percentages — another reason for avoiding excessive 

 moisture contents in a field soil. 



As has already been noted, the warming-up of a soil 

 becomes less and less rapid as the subsoil is penetrated. 

 This is not due to lessened conductivity, but rather to 

 a lessened heat supply. Bouyoucos f has shown that 

 under natural conditions the tendency of heat is to travel 

 downward more rapidly than laterally, due to a higher 

 moisture in the lower depths of the average field soil. 

 The time-temperature curves and the temperature gradi- 



1 Bouyoucos, G. J. An Investigation of Soil Temperature. 

 Michigan Agr. Exp. Sta., Tech, Bui. 17, p. 25. 1913. 



