240 NATURE AND PROPEETIBS OF SOILS 



carefully controlled conditions, lie found that unless excessive 

 amounts of manure were added no appreciable effects were 

 observed. Such results indicate that the heat of decay and 

 fermentation has little practical effect in modifying the tem- 

 perature of field soils. Without doubt there are certain local- 

 ized influences, but how important they may be is beyond 

 our present knowledge. As far as heat relations are con- 

 cerned, it seems that organic matter exerts its greatest effects 

 through a darkening of the color and an increase in the mois- 

 ture capacity of the soil. 



126. Loss of heat — conduction, radiation, and evapora- 

 tion. — ^Although small amounts of heat may be carried from 

 the soil by percolating water, the only important loss is into 

 the atmosphere above. This loss occurs in three ways, con- 

 duction, radiation, and evaporation. The loss due to evapora- 

 tion is easily the least important of the three. Conduction 

 and radiation have much to do with climatic control, since 

 the atmosphere receives its energy in large degree from the 

 earth rather than directly from the sun. Conduction from 

 soil to air and vice versa can be modified but to a slight extent 

 by man, a fortunate provision of nature. 



Terrestrial bodies are continually radiating energy waves 

 into the atmosphere, the change of temperature depending on 

 whether the receipt of such oscillations exceeds or falls short 

 of the loss. In the case of the soil, there is a very great dis- 

 sipation of energy in this way, radiation with conduction 

 being important climatic controls. The rapid changes in air 

 temperature are often directly due to these phenomena. 



These energy waves of terrestrial origin are very long,^ 



being within the infra-red group and consequently make 



no impression on the eye. They are often spoken of as the 



dark rays. Their energy capacity is higher than that of 



shorter oscillations. The trapping of heat in a greenhouse 



* Terrestrial bodies at ordinary temperatures give out waves varying 

 in length from .000270 to ,001500 cm. The warmer the body, the shorter 

 the wave length. 



