THE BIOLOGICAL CONDITIONS IN THE SOIL 107 



however, mainly confined to the surface layer ; the passage of heat 

 through the soil is slow and consequently the fluctuations at a depth 

 of 3 inches are less marked, especially in dry, loose soils. The thermal 

 conductivity of a soil is increased by moistening and by compacting. 



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, except- 

 ing 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 3 or more inches deep. 



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

 uniform in temperature. 



(5) At a depth of 4 to 12 inches soil is generally a little cooler than 

 the air in summer, and warmer in winter. 



Continuous records over periods of some months have been pub- 

 lished by Wollny (319) and by Thiele (282). English data are rather 

 scanty and generally refer only to 6 inch or 1 2 inch readings ; they 

 have, however, been collected and worked up by Mawley and by 

 Mellish.i 



Table XLV. — Temperatures of Soil at Different Depths under Varvino 

 Conditions. Russell. 



Effect of Weather. 



Effect of Vegetation. 



' See Quart. Journ. Roy. Meteor. Soc, i8gg, xxv., 238-65. 

 8* 



