42 



AGRONOMY 



every square yard of surface. A man of average size lying on 

 the earth will receive more heat in one hour than is needed to 

 raise a gallon of water to the boiling point. The same heat 

 would raise the temperature of a layer of soil, half an inch 

 thick, nearly 90 degrees. 



Variations in temperature. The changes of the seasons and 

 the alternations of day and night cause the temperature of 

 middle latitudes to range from many degrees below zero to 

 more than a hundred above in the course of a year. These 

 changes affect the soil for some distance downward, though 



in most parts of the United 

 States the difference between 

 the day and night tempera- 

 tures is not perceptible three 

 feet below the surface, and at 

 seventy-five feet below, sum- 

 mer's heat and winter's cold 

 are alike unknown. This ex- 



J I 



FIG. 19. Diagram to show the distri- 

 bution of the sun's rays 



When the sun is near the horizon, the 



same amount of heat is spread over a 



greater area than when it is overhead 



plains the fact that water 

 from deep wells is cool even 

 in summer. In the tropics 

 this point of unvarying tem- 

 perature, both for day and 

 night and for the seasons, is little more than a foot below the 

 surface. Much of the heat received by the soil is used in 

 evaporating the water in it. For this reason wet soils are 

 cold soils. The heat used in evaporating one pound of water 

 would warm 7500 pounds of soil one degree. Some of the 

 heat falling on the earth is also reflected back and serves to 

 increase the temperature of the air. 



Other factors that modify temperatures. The temperature 

 of the soil is also affected by its color, slope, and composition. 

 Southern slopes are warmer than northern slopes because they 

 receive the more direct rays of the sun. Soils sheltered from 



