300 SOILS: PROPERTIES AND MANAGEMENT 



entirely obscure in most cases the variations due directly to 

 buch factors as apparent specific gravity and humus con- 

 tent. Organic matter, because of its high water capacity, 

 usually accentuates the dominance of moisture in this 

 respect. While a humous soil of low volume weight may 

 warm up most easily when dry, its high water content may 

 so increase its thermal capacity as to markedly retard 

 its temperature changes. This is exemplified by Petit ^ 

 and Bouyoucos^ in their study of frost penetration in 

 peat. This soil was the last to freeze in winter and, 

 conversely, the last to thaw in spring. The advantage 

 of removing excess water by drainage is of importance 

 from this standpoint, as a wet soil is necessarily a colder 

 soil in spring than one that is well drained. This at least 

 partially accounts for the fact that a sandy soil is usually 

 an early one, and is therefore of particular value in truck- 

 ing operations. 



215. Absorptive power of soils for heat. — The greater 

 proportion of the heat received by the soil is obtained 

 by direct radiation from the sun. This radiant heat is 

 propelled by free wave action in the ether, the space 

 intervening between the sun and the earth being but 

 little affected by the transfer. Were the total amount 

 of heat received from a vertical sun by any unit surface 

 wholly absorbed by a layer of soil twelve inches thick, 

 the temperature of the soil would rise thirty degrees 

 Fahrenheit an hour. Such is not the case under normal 



1 Petit, A. Uatersuchungen liber den Einfiuss des Frostes 

 auf die Temperaiurverlialtnisse der Boden von VerseHedener 

 Physikoliselier Besehaffenheit. Forsch. a. d. Geb. d. Agri.- 

 Physik, Band 16, Seite 285-310. 1893. 



^Bouyoneos, G. J, An Investigation of Soil Temperature. 

 Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 214. 1913. 



