292 SOILS: PBOPJEBTmS AND MANAGEMENT 



soil gases and may greatly facilitate their movement. 

 This is essentially a physical relationship. It must be 

 kept in mind, however, that with heat as with other soil 

 factors, no clear-cut and distinct discussion of its effects 

 in one direction may be made without considering the 

 indirect influences that are continually opening up avenues 

 which lead to phases more or less foreign to the one under 

 discussion. This serves to emphasize the close correla- 

 tion of the various factors and conditions that must be 

 dealt with in a study of soils. 



209, Sources of soil heat. — The soil may receive 

 heat directly or indirectly from three general sources: 

 (1) from the sun, (2) from the stars, and (3) by 

 conduction from the heated interior of the earth. 

 The two last-named sources are so unimportant as 

 to warrant no further discussion, since the amount of 

 heat received by the soil therefrom is so small as to be 

 negligible. 



The sun, then, either directly or indirectly supplies 

 all the heat and energy that make it possible for soils to 

 support vegetation. This heat is derived in various ways, 

 as follows : — 



(1) By direct radiation of rays, both of light and of 

 invisible heat. These rays when absorbed tend to raise 

 the temperature of the absorbing medium. This source 

 of heat is by far the most important and may be desig- 

 nated as the direct method of heat induction. 



(2) A considerable amount of heat may be derived by 

 radiation and conduction from the atmosphere surround- 

 ing the earth. This heat has of course been originally 

 obtained from the sun and is passed on to the soil, the 

 length of the waves being somewhat changed in the transi- 

 tion, Clouds may sometimes serve as a blanket and shut 



