230 SOIL CONDITIONS AND PLANT GROWTH 



The temperature of the body of the soil depends on two 

 factors — the specific heat of the various layers of soil, and the 

 rate of propagation of the heat wave. At Rothamsted the 

 maximum temperature is commonly attained about 2. 1 5 p.m. 

 in the air, but at 5.30 p.m. at 6 inches depth in the soil. The 

 ease of propagation is not constant, e.g. it is much increased 

 by an increase in moisture content. This, however, much in- 

 creases the specific heat ; and the result of the two factors is 

 a considerable damping of the temperature wave as it pro- 

 gresses through the soil. In consequence, fluctuations in air 

 temperature of less than 2° C. are often inappreciable at 

 6 inches depth. 



The temperature curve of the soil at a depth of 6 inches 

 below the surface somewhat resembles that of the air in 

 summer, but it lacks the sharp peaks and depressions. The 

 soil minimum is always greater than that of the air, especially 

 in summer ; the maximum is also usually greater in winter, 

 although it is sometimes below in summer. In winter time, 

 however, the curve is often flat all the twenty-four hours and 

 sometimes shows practically no variation for two or three days 

 together (146^, Fig. 23). 



As a result of the interaction of these factors marked 

 temperature variations occur over comparatively small areas 

 of soil, being produced by differences in aspect, moistness 

 vegetation cover, looseness, and so on. Illustrations are 

 afforded in Table LX. 



For ordinary working purposes the following summary 

 will be found useful : — 



(i) A south slope is warmer than a north slope. 



(2) Bare land is warmer than land covered with vegetation, 

 excepting during winter months. 



(3) Soil exposed to the sun's rays is often hotter than the 

 air, and is subject to considerable temperature variations, 

 which, however, only slowly affect layers three or more inches 

 deep. 



(4) Moist soil, being a better conductor than dry soil, is 

 much more uniform in temperature. 



