144 RELATIONS OF SOIL TO HEAT 



occur without the intervention of living organisms are in 

 fact confined to certain limits of temperature, and are pro- 

 moted or retarded as the temperature approaches or recedes 

 from the optimum point, in a similar manner, though within 

 far wider limits, as we have already seen happens in the 

 case of the chemical changes produced by the agency of 

 life. 



Thus, speaking generally, the whole of the processes 

 within the soil become more active as the temperature rises. 

 Winter is a time of sleep, summer is a time of activity. The 

 productive power of a soil depends largely on its temperature. 



Sources of Soil Heat. The surface of a globe moving alone 

 in space must very shortly reach a condition of intense 

 cold. The present temperature of the surface of our earth 

 is almost entirely maintained by the radiation received from 

 the sun. To a very small extent the temperature of the 

 soil will be due to the heat evolved in chemical and 

 physical actions ; this heat is, however, in most cases merely 

 a reappearance of solar energy previously consumed in the 

 production of chemical or physical work, which afterwards, by 

 a reverse action, is resolved into its original elements. To 

 a small extent, the temperature of the surface of the earth is 

 also raised by the gradual outward passage of internal heat. 



The great internal heat of the earth is a familiar fact, 

 shown by the rise of temperature in the rocks when mines 

 are sunk into the earth, and also by volcanic phenomena. The 

 rate of increase in temperature on sinking below the surface 

 is not uniform in every place. If we assume as an average 

 a rise of i F. for each 50 or 60 feet of descent, we shall 

 have a temperature equal to that of boiling water at about 

 a mile and a half below the surface. It is impossible to say 



