TEMPERATURE 



301 



liberated. When water evaporates from a soil, the larger part of 

 the heat used in the process is taken from the soil. This has a 

 tendency to lower the temperature, and hence wet soils do not warm 

 up rapidly in the spring and are spoken of as " late " soils. They 

 become warm only when the greater part of. the water has evaporated 

 or when properly drained. 



If one-half pound of water is evaporated daily from a square 

 foot of soil, 4H.'J. 3 heat units or 121,790 calories are required, the 

 larger part of which would be taken from the soil. If all of the heat 

 necessary for this were taken from a cubic foot of loam soil having 

 an apparent specific gravity of 1.25 and containing 20 per cent of 

 moisture it would lower the temperature 15.5 degrees F. 



Clays, peats, and undrained soils are cold and late partly because 

 of this evaporation. 



Anything that diminishes evaporation aid? in increasing the 

 temperature of soils. Mulches, windbreaks, and drainage decrease 

 evaporation, and hence increase temperature. The strong winds of 

 spring increase evaporation, hence tend to keep the soil cooler until 

 it becomes fairly dry, when it warms up rapidly. 



The effect of the wind upon evaporation has been well shown 

 by King, who determined the evaporation at 20, 150, and 300 feet 

 to the leeward of a hedgerow. The amount was 24 and 33 per cent 

 greater for 150 and 300 feet respectively than at 20 feet. When 

 the air came across a field of standing clover 780 feet wide the 

 evaporation was 30.1 per cent greater at 150 feet, and 40 per cent 

 greater at 300 feet than at 20 feet from the field. 



3. Drainage. The effect of drainage on temperature at differ- 

 ent depths is shown in the table. The soils are the same. Drainage 



The Effect of Drainage on Temperature n 



removes the gravitational or free water, thus lowering the specific 

 heat so that the same amount of heat applied will raise the tempera- 

 ture more than if the soil contained much moisture. 



