122 THE TEMPERATURE OF THE SOIL [chap. 



Soil Temperatures. 



The accompanying curves (Fig. 8), show the 

 monthly mean temperatures of the soil at 6 inches, 3 

 feet, and 6 feet respectively, as compiled from readings 

 taken at 9 A.M. at Wye during 1896, the soil being a 

 light well-drained loam under grass. It will be seen 

 that the variations in temperature diminish with the 

 depth: in fact a point is soon reached, about 50 feet 

 down, below which the effect of the gain or loss of 

 heat at the surface is inappreciable, and the tempera- 

 ture is constant from day to day, only increasing with 

 the depth, according to the well-known law. Each 

 curve cuts each other curve at least twice ; for a 

 certain period the upper layer is giving, and during 

 the rest of the year, receiving heat from the layer 

 above or below. The maximum temperature attained 

 at a depth of 3 feet comes a little later in the year 

 than the maximum for 3 inches, and the maximum 

 at 6 feet lags still further behind, owing to the slow- 

 ness with which the heat is conducted. It will be 

 seen that the curve indicating the temperature at 6 

 inches (and the mean figures for 3 and 9 inches are 

 almost identical) does not reach the 41 F. required for 

 the beginning of vegetative growth until April ; it 

 is, however, constructed from monthly averages only, 

 and from observations taken at 9 A.M., when the 

 surface soil has been considerably cooled during the 

 night. Much higher temperatures are obtained during 

 certain parts of the day even in the early spring 

 months, otherwise no germination could take place; 

 these diurnal and hourly fluctuations are, however, 

 chiefly confined to the surface soil. The following 



