32 BULLETIN" 355^ U. S. DEPAETMEjSTT OF AGEICULTUKE. 



Factors influencing soil temperature. — ^The sun is the chief source of 

 heat for the earth's surface. The sun's rays are conducted to the 

 earth as hght. These rays are transformed into heat and absorbed, 

 or are largely reflected back into the atmosphere, depending upon the 

 condition of the soil material which the rays reach. Dark soils trans- 

 form and absorb as heat much more from the sun's rays than do light- 

 colored soils. Besides the sun, an indhect source of a small amount 

 of heat is the chemical and microbiological changes taking place in 

 the soil. A chemical reaction usually produces heat, and microbio- 

 logical activities frequently do. 



The prmcipal conditions affecting the temperature of the soil are: 

 (1) Latitude. The farther north or south of the equator a land sur- 

 face is the less direct are the sun's rays upon it and, other things 

 being equal, the less will be the total heat absorbed in any given 

 time. (2) Slope. A southern hihside will be warmer than the 

 northern, because the sun's rays upon it are more direct. (3) Cir- 

 culation of air above the soil. The varying temperature and hu- 

 midity of the cuiTents of air upon hillsides and in valleys have a 

 considerable effect upon the temperature of the soil areas over which 

 they pass. (4) Composition and texture of the soil. Both of these 

 factors affect the conductivity of heat into the subsurface soil. 

 Some rock materials are better conductors of heat than others. 

 Again, air is a poor conductor of heat, and the greater the pore space 

 in son the less rapidly will heat be conducted through it. Fine- 

 textured soils thus conduct heat less rapidly than coarse-textured 

 soils of hke composition. Clay soils warm up less quickly in spring 

 than sandy soils which have less pore space. Peat soils formed in 

 marshes are very open and spongelike, and this large air space causes 

 heat to pass down into such soils with extreme slowness. Frost is 

 often found in marshes several weeks after it has entirely disappeared 

 in upland and more compact soils. (5) Water content of the soil. 

 This has a very important influence upon the soil temperature. It 

 takes nearly twice as much heat to raise water 1° in temperature 

 as it does to raise the same weight of soil 1°. Then the evapo- 

 ration of moisture from the surface of the soil uses up a great deal of 

 heat and does much to keep the soil cold. It requires as much heat 

 to evaporate a pound of water as would raise the temperature of a 

 cubic foot of average soil over 10° Fahrenheit. (6) Color. Dark- 

 colored soils, other conditions being equal, are warmer than Hght- 

 colored. 



There are at least four practical means by which the temperature 

 of soil may be regulated: (1) By means of vegetable matter. A good 

 supply of barnyard manure or green manure in the soil wiU have an ap- 

 preciable effect in warming it. (2) ByroUing. The heat conductivity 



