THE PHYSICAL PEOPEETIES OF SOILS 79 



The presence of organic matter in the soil therefore 

 increases the capacity for heat, but, as it rarely amounts 

 to more than 10 per cent, of the dry matter, except in soils 

 of vegetable origin, its effect is not great. 



The capacity for heat of the dry matter of soils is usually 

 about 0*2, and any variation, due to different proportions 

 of organic matter or of the several mineral constituents, is, 

 as a rule, too small to sensibly affect the temperature of 

 the soil. 



Under natural conditions the capacity for heat of soils 

 depends almost entirely upon the quantity of water they 

 contain, and the variations due to this cause have an im- 

 portant effect on the temperature. For example, if a cubic 

 foot of soil contain 75 Ibs. solids 1 and 25 Ibs. water, and 

 absorb enough heat to raise the temperature 5, the same 

 amount of heat applied to the dry matter alone would have 

 raised the temperature 12'9. The water absorbs some of 

 the heat and the temperature is thereby reduced 7*9. Again, 

 equal amounts of heat would raise the temperature of 



75 Ibs. dry soil 10 C. 



75 " +10 Ibs. water .... 6'1 C. 

 75 ,, + 20 . . . . . 4-4 C. 

 75 +30 . . 3-4 C. 



In the last case the reduction in temperature due to the 

 presence of water is over 60 per cent. 



Soils which retain the smallest quantity of water coarse 

 sands have the lowest capacity for heat and attain the 

 highest temperature. Those which retain most water- 

 humous soils have the highest capacity for heat and are 

 the coldest. The capacity for heat of humus is greater 

 than that of the minerals, but owing to the difference in 

 density the capacity for heat of equal volumes is about 

 the same. 



1 Specific heat 0'21. 



