166 RELATIONS OF SOIL TO HEAT 



When we compare the specific heats of equal volumes, we 

 find that water is still far ahead of all the other soil 

 constituents, but the numbers for humus, clay, calcium 

 carbonate, and quartz have become nearly equal. The same 

 quantity of heat would raise to the same temperature 2 cubic 

 feet of quartz, and i cubic foot of water. If soils were of the 

 nature of a solid rock, without interstices, and without 

 moisture, they would very uniformly have a specific heat 

 of .50 55 when compared with their own volume of water, 

 unless indeed much iron or magnesium were present. 



We have already seen that the water-holding power of a 

 soil, viewed in its relation to fertility, is far more accurately 

 stated per unit of volume than per unit of weight (p. 69) ; 

 the same may be said of the relations of soil to heat. It 

 is the bulk, or depth of the soil that is warmed by the sun, 

 and not the weight, which is important to the plant. To 

 become acquainted with the relations of natural soils to heat 

 on this basis of volume or depth, we must clearly take into 

 account the varying proportions of the bulk which consist of 

 the spaces between the particles; we must also take into account 

 the proportion of water which will be normally present in 

 the soil. The information already given as to the weight of 

 different soils per cubic foot, and as to the proportion of water 

 contained by various soils, both when in an air-dry and in 

 a drained condition, will enable us to calculate the specific 

 heat of soils per unit of volume in various natural conditions. 



If, in the first place, we regard soils in the air-dry state, 

 that is containing only hygroscopic water, we find that there 

 is little difference in their specific heat per unit of volume, the 

 coarsest sand, the purest clay, and an air-dry peat, all having 

 specific heats varying from about -30 to -42, if the specific 



