SOIL AND CLIMATIC ADAPTATIONS OF CORN 215 



prevails there is sufficient sunshine for corn production 

 up to 70 degrees latitude. 



263. Influence of temperatixre. — ®orn requires, in 

 addition to a moderately large, well-distributed seasonal 

 rainfall and a large amount of sunshine, a relatively high 

 temperature. WMle it is difficult to give precise limits to 

 any influence that is one of several absolutely necessary, 

 the direct relation between temperature and yield is more 

 obscure than that between rainfall and yield. In fact a 

 high average temperature and large precipitation are 

 somewhat opposed to each other, as low rainfall during 

 the growing season is usually accompanied by a high 

 average temperature. It is the temperature • during the 

 corn growing season, inclusive, rather than the average 

 annual temperature that influences the jdeld of the 

 crop. Three-fourths of the total corn crop of the United 

 States is produced between the July isotherms 70° F. 

 and 80° F. 



264. Length of growing season. — From the stand- 

 point of the farmer there is no factor in the study of cli- 

 mate that should be given more consideration than the 

 average length of the growing season. It serves as a key 

 to accurate knowledge relative to the possibiUties of suc- 

 cess or failure ia the production of crops. Fig. 16 shows 

 the average length of the crop growing season in the cotton- 

 belt to vary from 200 days in the northern limit to 300 

 days in the southern limit. As we proceed north from the 

 cotton-belt the growing season continues to decrease in 

 length. These figures are based on the average dates of 

 the last killing frost in the spring and the first killing frost 

 in the fall. As a matter of fact, the actual length of the 

 growing season is most often limited by factors other than 

 frost. In the cotton-belt the growing period is usually, 



