32 BULLETIN 355,, U. S. DEPARTMENT OF AGRICULTURE. 
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 light. These rays are transformed into heat and absorb ed, 
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 indirect 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 principal 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 hillside 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 currents 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 soil the less rapidly will heat be conducted through it. Fine- 
textured soils thus conduct heat less rapidly than coarse-textured 
soils of like 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 light- 
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 will have an ap- 
preciable effect in warming it. (2) By rolling. The heat conductivity 
