224 



SOIL PHYSICS AND MANAGEMENT 



plant, but not so much as to crowd out the oxygen, which is 

 equally essential. 



(e) The temperature of the soil will be raised by the removal 

 of the water, since the specific heat of the soil will be lower with 

 less water. If the specific heat of water is 1 and that of soil is 

 0.2, then a waterlogged soil having an apparent specific gravity of 

 1.2 and 50 per cent of moisture would have a specific heat of 0.46, 

 or the amount of heat required to raise the temperature of the wet 

 soil one degree would be more than twice as great as for the dry 

 soil. Another factor that affects the temperature is the lowering 

 of evaporation by drainage. Evaporation is a cooling process, and 

 every pound of water evaporated from the surface of the soil re- 

 quires 966.6 heat units, and this will be taken largely from the 

 soil. Hence wet soils are "late" soils. They may be trans- 

 formed into " early '* ones by drainage (Fig. 95). 



Drainage lengthens the growing season of certain soils, and 

 may possibly permit a complete change of crops. Conditions are 

 more favorable for biological activity in the drained soil because 

 of the increase in temperature and of better aeration. King found 

 that well-drained sandy loam had a temperature of 66.5 degrees F., 

 while in an undrained black marsh the temperature was 54 degrees 

 at the same depth. 



Experiments conducted with trays filled with the same soil, 

 one of which was drained while the other was not, show differences 

 as given in the table. 



Effect of Drainage on Temperature of a Soil l Degrees Fahrenheit 



It will be noted that the greatest difference between the drained 

 and undrained soil at one inch in depth was 13.7 degrees, at two 

 inches 9 degrees, and at 4 inches 6.2 degrees. 



