PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 255 



a correlation with the corn yield and thus show this period to 

 be the most critical one in the development of this crop, in the 

 United States. 



THE CRITICAL PERIOD FOR POTATOES 



For the State of Ohio the most critical calendar month for 

 potatoes is July, as has been shown in an article in the Monthly 

 Weather Review for May, 191 5. 



Table 7. Correlation Between the Temperature and Rainfall and 

 THE Yield of Potatoes in Ohio. 1860 to 1914. 



Correlation Coefficient (r) 



Period. Temperature. Rainfall. 



April —0.21 



May —0.10 0.06 



June —0.22 0.10 



July —0.51 0.33 



August —0.31 0.22 



September —0.21 —0.13 



October —0.11 0.07 



June and July combined — .50 



July and August combined — 0.50 0.87 



June, July and August combined — 0.49 



In table 7 it is shown that cool weather is desirable during 

 each month of the svuTimer to produce the best crop of potatoes, 

 while July is the important month. In fact this is the only 

 month with a correlation coefficient high enough to warrant any 

 decided argument in the matter. When the summer months are 

 combined the results are not far from those for July alone. The 

 higher value of r shows that the temperature has a greater effect 

 in Ohio than rainfall, on the potato crop. 



Figure 4 is a dot chart giving the influence of July rainfall 

 and temperature upon the potato crop in Ohio. It covers a 

 period of 55 years. This shows that warm weather in July is 

 nearly always unfavorable for potatoes and that there is seldom 

 a good yield when July is warm and wet. This is in marked 

 contrast to the effect of these conditions on the corn crop. 



When the mean temperature in July has averaged more 

 than I degree a day higher than the normal the yield of potatoes 



