298 SOILS: PROPEBTIES AND MANAGEMENT 



a reduction of specific heat thereby occurs. Under such 

 conditions more heat is necessary to raise the temperature 

 of the sand than is the case with the weight expression. 

 This is because of its high apparent specific gravit} . The 

 clay shows \ery Httle change, as its apparent specific 

 gravity is about one; but the humus exhibits a marked 

 falhng-off, due to its exceedingly low volume weight. 

 The factor that tends to vary the specific heat of dry 

 soil under natural conditions, therefore, is the apparent 

 specific gravity, or the volume weight. By deep and 

 efiicient plowing the farmer may encourage the warm- 

 ing of his soil, due to a lowered thermal capacity. By 

 increasing its humus content he may attain the 

 same result, since the volume weight is depressed to 

 a markedly greater extent than the specific heat is in- 

 creased by the addition of organic matter. In fact, any 

 operation on or any addition to the soil that will vary 

 its apparent specific gravity will in turn affect the specific 

 heat. 



214. Effect of water on specific heat. — One other 

 factor, much more potent than the two already men- 

 tioned, is yet to be discussed. This factor is water, so 

 universally present in soils and of the greatest importance 

 in all natural soil phenomena. As the specific heat of 

 water is very high compared with the thermal capacity 

 of the soil constituents, any addition of it must naturally 

 raise the specific heat of a normal soil. That moisture, 

 not apparent specific gravity nor organic content, is the 

 controlling factor is demonstrated from the following 

 data, calculated by Ulrich ^ on a volume basis : - 



1 Ulrich, R Untersuehungen ixber die Warmekapazitat der 

 Bodenkonstituenten. Forseh a. d Geb d Agri -Physik, Band 

 17, Seite 27. 1894. 



