80 SOILS AND MANURES 



The following table shows the capacity for heat of equal 

 masses of soils and soil constituents : 



Water . I'OO Loamy soil (dry) . . . . -21 



Humus . '48 same with 10% organic matter . -24 



Clay . . '23 Loamy soil 10% water . . . -29 



Limestone. '21 ,, ,, 20% ,, -37 



Quartz . -19 30% ,, -45 



Ferric oxide *16 



The capacity for heat of equal masses multiplied by the 

 density gives the capacity for heat of equal volumes. 



Effect of Evaporation of Water on the Temperature of 

 Soils. Some of the heat absorbed by the soil is dissipated 

 by the evaporation of water. During the summer months 

 the daily evaporation may amount to about a quarter of a 

 pound or more of water per square foot of surface. The 

 latent heat of water, i.e., the heat of vaporisation, at the 

 ordinary temperature, is 588 units. The evaporation of 

 a quarter of a pound of water would therefore absorb 

 enough heat to reduce the temperature of 147 Ibs. of water 

 1 C. If this amount of heat were distributed over one 

 cubic foot of soil containing 75 Ibs. of solid matter and 

 25 Ibs. of water the cooling effect would be equal to a 

 reduction of temperature of 3'6 C. If the soil contained 

 less water the cooling effect would be greater ; with half 

 the quantity of water the cooling effect would be equal to 

 a reduction of 4*5 C. of temperature. 



Evaporation generally takes place most rapidly when the 

 temperature of the soil is actually rising under the 

 influence of the sun's rays, but if the evaporation were 

 prevented the increase of temperature would be so much 

 greater. Any process which hinders evaporation, such as 

 loosening the surface or covering it with manure, etc., 

 tends to conserve the heat and sustain the temperature. 



Radiation of Heat. Soils cool down at night by radiat- 

 ing heat from the surface, The speed of radiation depends 



