168 RELATIONS OF SOIL TO HEAT 



drained, which are always chosen as specially suitable for 

 early garden crops (pp. 20, 57) ; the reason of this suitability is 

 now apparent they become warm in spring time earlier than 

 any others. 



The influence which oxide of iron has on the specific heat 

 of soils demands further study. The oxide referred to in 

 Table XXVIII was the anhydrous mineral, while the hydrated 

 oxide is the form usually present in soils. 



Tillage, by loosening the soil, and thus diminishing the 

 weight in a given volume ; and also by diminishing the pro- 

 portion of water retained at the surface, tends to decrease the 

 specific heat per unit of volume, and so far facilitates the 

 warming of the soil. We shall see however presently that 

 tillage diminishes the power of the soil to conduct heat. 



Conductivity of Soil Constituents. The temperature of 

 both the surface soil and subsoil depends in part on the 

 power of conducting heat which the soil possesses. A soil 

 having little power of conducting heat l will become very hot 

 on the surface when exposed to the sun during the day, and 

 the surface will become very cold during a clear night when 

 heat is lost by radiation. A soil having a greater power of 

 conducting heat will be warmed to a greater depth by the sun 

 during the day, and the surface will be longer in cooling 

 during the night, as heat will then travel to the surface from 

 the interior. Good conduction thus tends to equalize the tem- 

 perature of the surface soil, while it occasions a greater range 



1 The ' conduction of heat ' is throughout this section used in its popular 

 sense, which would perhaps be more accurately expressed as the ' propagation 

 of temperature.' The quantity of heat transmitted, and its facility of move- 

 ment, cannot be measured by the rise of a thermometer at a distance from 

 the source of heat. A smaller rise of the distant thermometer may be 

 attended with a greater transmission of heat if the specific heat of the 

 intervening matter is higher than in the comparative instance. 



