150 RELATIONS OF SOIL TO HEAT 



Experiments by F. Wagner on the same subject (Wollny, 

 Forsck. der Agrikulturphysik, v. 373) showed that with 

 the heaviest dressing of farmyard manure, about twenty 

 tons per acre, the temperature of the soil remained above 

 that of the unmanured land for four to twelve weeks. The 

 greatest excess of temperature observed was 5F., but the 

 highest average excess of temperature was i F. Generally 

 the average excess of temperature by manuring was from 



0'2-0-7. 



The ploughing in of green crops has a similar effect on 

 the temperature of the soil as the application of farmyard 

 manure. The after result of such organic manuring is to 

 somewhat lower the temperature of the soil, probably for 

 the reason already stated. 



For the greatest rise in temperature to occur, a soil must 

 be porous, and moist, and of a temperature not below 50 F. ; 

 the rise is in fact greatest in summer time, all processes of 

 fermentation and oxidation being then most vigorous. 



As tillage greatly promotes oxidation, the cultivation of 

 a soil in spring, especially if it contains vegetable residues, 

 (as for instance a clover ley), will tend to increase its 

 temperature. 



We have seen that neither the internal heat of the earth, 

 nor the chemical and physical changes which occur in the 

 soil, have any considerable effect on the temperature of the 

 earth's surface; this depends almost entirely on the amount 

 of radiant energy received from the sun. 



The amount of heat received from the sun depends of 

 course primarily on the activity of solar processes, which 

 may apparently vary from time to time. The great altera- 

 tions in climate in the past history of our globe are difficult 



