Investigations at Peru, Nebraska. 



37 



soil was quite too low for maximum root development during the cold rainy 

 month of April (cf. Lehenbauer, 1914). During May, a slow but uniform 

 rise occurred, while differences between the temperatures on June 9 and 23 

 were quite pronounced. This ushered in a hot, dry period which extended 

 to July 4. It is interesting to note that the deeper soils into which the new 

 roots were extending were progressively colder. Undoubtedly temperature 

 has a marked influence upon crop-root development under field conditions. 



April 



14 



May 



12 20 



June 









































/ / 



















3" 



















/ / 

 / / 



3' 



r s 







'//'/ 



% 



































Fig. 15. — Soil-temperatures at various depths during 1919. 



May June 



5 12 19 26 2 9 16 23 



July 



August 



15 22 29 7 12 20 25 



Fig. 16. — Average day and night temperatures (solid lines) 

 and average maximum and minimum temperatures. 



Indeed, it has been suggested (Shepperd, 1905) that in northern latitudes, 

 where the ground freezes deep in the winter, the soil may be too cold for the 

 roots of cereals to penetrate to a depth greater than 3 feet before midsummer 

 (cf. Pulling, 1918). 



The average weekly day and night temperatures are shown in figure 16, 

 which also includes the average weekly maximum and minimum tempera- 

 tures. The average for the day temperatures was determined from the 



