140 



THE TEMPERATURE OF THE SOIL [chap. 



one foot, except in the case of heavy precipitation and 

 a pervious soil. It will also be noticed from the 

 last curves that during part of the day the temperature 

 at the depth of 6 inches ran up to a point well above 

 the minimum required for germination, although, the 

 mean soil temperature at 9 A.M. was near that limit. 



Temperatures required for Growth. 



Reference has already been made to the fact that a 

 certain temperature is necessary before the vital 

 processes involved in growth become active ; this 

 temperature is not always the same, but may be con- 

 sidered to lie between 40 and 45 F. for most of the 

 plants grown as crops in this country. 



The following table shows minimum, optimum, and 

 maximum temperatures of growth for a few plants : 



The next table shows the effect of soil temperature 

 upon the growth of the root of maize 



Root Growth of Maize in 24 Hours. 



