314 SOILS: PROPERTIES AND MANAGEMENT 



moisture content for plant growth, will place the oil in 

 the best possible condition, consistent with plant devel- 

 opment, for good heat movement. 



224. Effect of evaporation of water on soil temperature. 

 — There is perhaps no factor, besides the loss of heat by 

 direct radiation, which exerts such an effect on soil tem- 

 perature as does evaporation. The fact that water does 

 not allow the long rays received by direct radiation to pass 

 readily through it accounts for its rapid vaporization. 

 This evaporation, caused by an increased molecular 

 activity, requires a certain expenditure of heat, result- 

 ing in a cooling effect on the water and consequently on 

 any material in close contact with it. To evaporate a 

 pound of water requires the withdrawal of about 966 

 heat units. 1 This is sufficient to lower the temperature 

 of a cubic foot of saturated clay soil about 10° Fahrenheit. 

 The difference in temperature exhibited by wet and dry- 

 bulb thermometers measures the cooling effect of 

 evaporation. 



Any condition that increases the rate of evaporation 

 lowers the temperature of the surface concerned. The 

 amount of water present is undoubtedly the controlling fac- 

 tor in this regard." King 2 found, in his study of a drained 

 and an undrained soil in April, that the former maintained 

 a temperature ranging from 2.5° to 12.5° Fahrenheit 

 higher than the latter. Parks 3 records the same general 



1 An English heat unit is the amount of energy necessary to 

 raise one pound of pure water from 32° to 33° F. It is equal to 

 about 778 foot-pounds. 



2 King, F. H. Physics of Agriculture, p. 220. Published 

 by the author, Madison, Wisconsin. 1910. 



3 Parks, J. On the Influence of Water on the Temperature 

 of Soils. Jour. Roy. Agri. Soc. Eng., Vol. 5, pp. 119-146. 

 1845. 



