146 THE TEMPERATURE OF THE SOIL] [chap. 



that the covering of soot warmed the soil until at 3 

 P.M., when the maximum temperature was attained, 

 the difference was 2-4 ; this superiority is also re- 

 tained during the later cooling stages ; even at 9 p.m. 

 the blackened soil was still 2-5 warmer than the bare 

 ground. The whitening with lime had caused so con- 

 siderable a reflection of the radiant heat that the soil 

 beneath was always 2 to 3 cooler than the bare ground. 

 In carrying out this experiment it is necessary to use no 

 more lime or soot than will distinctly colour the soil ; 

 the results will be disturbed if an excess of either loose 

 powder acts as a mulch. 



Specific Heat. 



The specific heat of the substances of which soil , 

 is composed is comparatively low, ranging from 01 

 to 0-2, i.e., only from one-tenth to one-fifth as much 

 heat will be necessary to raise the temperature of 1 

 lb. of dry soil by i, as would be required to produce 

 the same rise of temperature in an equal weight of 

 water. The humus possesses the greatest specific heat 

 and the sand the least ; against this must be set off the 

 fact that the densities of these soil constituents vary 

 in the opposite sense, so that the amounts, of heat 

 required to bring about a given rise of temperature to 

 a certain depth in different soils are more nearly equal. 

 The specific heats are, however, small in every case 

 when compared with that of water ; hence soils which 

 retain much water will require far more heat to raise 

 their temperature than dry soils would. In consequence, 

 clay and humus soils are cold because the water they 

 retain gives them a high specific heat, they require more 

 of the sun's rays in spring to bring them up to the 

 proper temperature for growth, while sandy and other 

 open-textured soils are warm because of their dryness. 



