426 



Irrigation and Drainage 



similar to this are often observed early in the spring, 

 when leaves are just unfolding. A strong drying 

 wind on a cool day, with the soil also cold, withers 

 the leaves much as if they had been frosted. 



The germination of seed is very much influenced 

 by the temperature of the soil, maize requiring 16 

 days to appear above the ground when the soil tem- 

 perature is 60 F., or below, when if the warmth is 

 72 or above, 3 days or less will do the same work, 

 besides giving much stronger plants. These effects 



Fig. 131. Influence of soil temperature on the rate of germination of maize. 



of soil temperature are clearly demonstrated in Fig. 

 131. Indeed, it will often happen that when seed 

 of rather low vitality is planted in a soil a little too 

 cold, germination will not take place at all, or if it 



