238 



JOHN ERNST WEAVER 



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record are remarkably uniform in height, as are also the depres- 

 sions uniform in depth. The difference between the crests and 

 the depressions is shown in figure 4 where the average weekly 

 minimum and maximum soil temperature (solid lines) are 

 recorded, since these are more important to plant growth than 

 the mean. The differences between these averages increase 

 from about 10°F. (at 4 inches depth) April to June, to about 24°F. 

 (at 3 inches depth) in July and August. For purposes of com- 

 parison graphs representing the average weekly maximum and 

 minimum air temperatures (at 3 inches above the soil surface 

 and in the shade) recorded at the same station are also given in 

 figure 4. 



TABLE 4 

 Soil temperatures at a depth of 3 inches 



N.E. slope 

 S.W. slope 

 N.E. slope 

 S.W. slope 

 N.E. slope 

 S.W. slope 



July 27 

 July 27 

 July 28 

 July 28 

 July 29 

 July 29 



It is interesting to note that, while the soil temperatures at 

 night were always warmer than those of the air, on many days 

 during June, July, and August the air temperatures were lower 

 than those of the soil. 



Owing to the lagging of the soil temperature the maximum 

 was not attained until about 4 p.m. and the minimum at 7 a.m. 

 as compared with 3 p.m. and 5 a.m. respectively, for maximum 

 and minimum air temperatures. 



The seasonal march of soil and air temperatures varies inversely 

 with the soil water. That is, the temperatures steadily become 

 higher in proportion as the soils become drier. This condition 

 occurred on both slopes, but the temperatures were lower and 

 the extremes of variation less on the sheltered side. A single 

 example of maximum and minimum soil temperatures on the 

 two slopes and at 3 inches depth must suffice here. 



