PHYSICAL INTER-RELATIONS IN SOILS 33 



the particles were first loosened by being rolled over each 

 other, and then rolled into crumbs to be held together by 

 films of water. If the soil had been over-dry, the plow 

 could not have caused the soil particles to separate and 

 be re-crumbed. (Compare Figs. 19 and 20.) 



49. Sources of soil heat. The sun is the great source 

 of heat, and its heat comes to the earth as radiant energy. 

 A part of the sun's heat is intercepted by clouds and other 

 air moisture ; a part of it is thrown back into the atmos- 

 phere from the earth's surface. Some heat comes from 

 the interior of the earth and, under certain conditions, 

 plays an important part in the behavior of soils. 



50. Capillary water and soil temperature. Mention 

 has already been made of the importance of heat in crop- 

 growing, and reference has been made to Table IV showing 

 the average temperature of soils in different localities for 

 the growing months. Capillary water plays a large and 

 important part in modifying soil temperatures. 



51. Specific heat of soils. The amount of heat re- 

 quired to raise the temperature of a pound of soil one 

 degree, as compared with the amount of heat required to 

 raise the temperature of a pound of water one degree, is 

 called the specific heat of that soil. The specific heat is 

 more concisely defined as the " number of calories needed 

 to raise one gram of a substance 1 C." Here, however, 

 metric units are used, though the specific heat values 

 obtained will be the same. 



Bouyoucos, working with Michigan soils, 1 found : 



The specific heat of sand to be .1929. 



The specific heat of clay to be .2059 approximately . 



1 Page 495, Report of Michigan State Board of Agriculture, 

 1913. 



