156 RELATIONS OF SOIL TO HEAT 



and October the south side; from November to April the 

 south-west. A south-east aspect is the one generally pre- 

 ferred by gardeners, as the sunshine in this case begins at 

 an earlier hour. Such an aspect may, however, be attended 

 with disadvantage in the case of spring frosts, owing to the 

 too rapid thawing of the vegetation. 



King (The Soil, 228) determined the temperature of a red 

 clay soil on the south shore of Lake Superior on Ju]y 31, 

 both where the slope was 1 8 and on the level. The results 

 were as follows: 



Firstfoot. Second foot. Third foot. 



South Slope 70.3 ... 68.l ... 664 F. 



Level 67-2 ... 654 ... 63.6 



Excess on South 3-l ... 2. 7 ... 2.8 



Thus, in the middle of summer, the greater heat on the 

 south side had penetrated to a considerable depth. 



When planting in rows or ridges the exposure to the sun 

 should always be considered. Rows running north and south 

 will be equally exposed to the sun on both sides, while those 

 running east and west will, in the case of tall plants, receive 

 but little sunshine on their northern side. 



Temperature of Surface Soil. In the case of a bare dry 

 soil, freely exposed to the sky, the range of temperature 

 at the surface is very great, far greater than that of the 

 air above it. The maximum temperature reached by such 

 a soil is chiefly determined by the intensity of the solar 

 radiation which it receives. Sir J. F. W. Herschel sunk a 

 thermometer 4 inches deep in the sand in South Africa, and 

 observed the temperature rise to 159 F. Schiibler (Jour. 

 Roy. Agri. Soc. 1 840, i. 206) determined during two years the 

 temperature of the garden mould on the south side of his 



