SOIL HEAT 



29 r 



therefore show different weights to the cubic foot. A 

 specific heat comparison based on weight, therefore, does 

 not yield a fair idea of the heat capacities of two soils. 

 The multiplication of the weight specific heat by the 

 apparent specific gravity of the soil in question will 

 obviously yield a volume specific heat, which is a 

 much more rational basis for comparison. A quota- 

 tion from Ulrich 1 makes clear the value of such a com- 

 putation : — 



Specific Heat of Soil Expressed by Weight and by Vol- 

 ume of Soil 



Sand 

 Clay 

 Humus 



Apparent 

 Specific 

 Gravity 



1.52 



1.04 



.37 



Specific Heat 

 by Weight 



.1909 

 .2243 

 .4431 



Specific Heat 

 by Volume 



.2901 

 .2333 



.1639 



It is evident that in the first case the specific heat is 

 governed by the organic content of the soils in question; 

 the greater the amount of organic material present, the 

 higher is the thermal capacity. Such is not the case when 

 the specific heat of the soil is calculated on a volume basis. 

 In an expression of the thermal capacity on this rational 

 basis, namely, that of volume, the apparent specific grav- 

 ity, or volume weight, is the dominant factor. The ad- 

 dition of humus when this method of expression is em- 

 ployed merely serves to lower the volume weight, and 



1 Ulrich, R. Untersuchungen iiber die Warmekapazitat der 

 Bodenkonstituenten. Forsch. a. d. Geb. d. Agri.-Physik, 

 Band 17, Seite 1-31. 1894. 



