SOIL HEAT 241 



is partially due to the tendency of the objects within the house 

 to give off these long rays, which do not pass through the 

 glass with the facility possessed by the shorter vibrations by 

 means of which a large proportion of the energy was intro- 

 duced. 



The texture, structure, and color of the soil have little in- 

 fluence on radiation. Moisture tends to hasten it a trifle, 

 since water is a better radiator than soil. Mulches, as they 

 are loose and dry, may check radiation slightly. Artificial 

 coverings, shelters, and clouds seem to exert the greatest effect. 

 It is often feasible to protect plants from frost by interfering 

 with radiation and conduction. Clouds by shutting in heat, 

 may in some cases prevent a frost that would otherwise occur, 

 due to the rapid cooling. Snow likewise has a protecting 

 effect and may often prevent the soil underneath from freez- 

 ing. While man may influence radiation locally, it is evident 

 that the total energy loss can be checked but little. 



The effect of evaporation on the temperature of the soil is 

 especially noticeable because of its rapid action. This vapor- 

 ization of water is caused by an increased molecular activity 

 and requires the expenditure of a certain amount of heat, 1 

 which results in a cooling effect on the water remaining and 

 consequently on the soil and air with which it is in contact. 

 It requires 267.9 kilogram calories to evaporate one pound 

 of water at 50° F. This is sufficient to lower the temperature 

 of a cubic foot of saturated clay about 20° F., providing that 

 all of the energy of evaporation comes from the soil and its 

 water. 



The low temperature of a wet soil is due partially to evapo- 

 ration and partially to high specific heat. King 2 found during 



1 It requires 536.6 gram-calories to evaporate one gram of water at 

 100 °C, while 596.7 calories are necessary if evaporation takes place at 

 0°C. The calories (C) required to vaporize one gram of water at any 

 temperature (t) may be calculated by the formula: 

 C = 596.73 — .601 t 



a King, F. H., Physics of Agriculture, p. 20; Madison, Wis., 1910. 



