SOIL HEAT 307 



Iii fact, the soil may be warmed by conduction of heat 

 from air to soil. This probably occurs to some extent 

 in spring, when the air is growing warmer, due to low 

 specific heat and its movement. The changes in air 

 temperatures are always more rapid and usually greater 

 in range, due to the factors cited above. 



219. Conductivity and convection of heat in soils. — 

 While radiation has to do with the transfer of heat by 

 ether waves, conductivity is a term used in relation to 

 molecular transmission of energy through the body under 

 investigation. It may be defined as the amount of heat 

 in calories that will pass across a cube of unit edge (1 

 centimeter), in unit time (1 second) under a temperature 

 gradient of 1 degree Centigrade. Convection refers to 

 the transmission of heat by actual apparent and visible 

 movements of matter. It is to these two modes of trans- 

 fer that we owe the possibility of the soil's warming below 

 a surface that receives most of its heat as radiant energy. 

 It must be remembered that in studying the soil we are 

 dealing with a material made up of mineral and organic 

 compounds and always containing, under normal condi- 

 tions, a certain quantity of water. Air likewise is always 

 present, which, while a poor conductor of heat, may carry 

 energy by convection. Besides these varying substances, 

 often in loose contact and usually containing air capable 

 of considerable movement, there is bound to occur a 

 certain amount of transfer resistance which is the heat 

 resistance found at the boundary of two substances in 

 contact. The study of heat movement downward through 

 a soil is difficult to analyze, since it is almost impossible 

 to control the factors concerned while varying any one. 

 In a normal soil this heat movement occurs through both 

 the agency of conduction and that of convection, depend- 



