SOIL HEAT 225 



The effect of heat on the physical changes within the soil 

 is often vital. The influence that temperature variation exerts 

 on percolation, evaporation, and capillary movement of soil- 

 water; on diffusion of gases, vapors, and salts in solution; 

 and on osmosis, surface tension and vapor tension phenomena, 

 may serve as examples of such heat modifications. Moreover, 

 successive freezing and thawing of the soil greatly aids in 

 granulation and aeration. The aspirating effect of a slight 

 change in temperature is so tremendous as often markedly to 

 renew the oxygen supply of the furrow slice. 



In order fully to understand the practical and scientific 

 relationships involved in even a partial control of soil heat, 

 a certain cycle of events must be recognized. The cycle be- 

 gins with the acquisition of energy from the sun and the 

 establishment of certain temperature relations which depend 

 on absorption activity and the facility with which heat is 

 transferred from place to place. The important chemical, 

 physical, and biological transformations within the soil de- 

 pend as much on such movements as on the intensity of the 

 temperature factors. Much of the energy so involved is soon 

 lost from the soil, returning again to the space from which 

 it came. Thus the cycle is completed, having provided the 

 temperature conditions necessary for successful crop produc- 

 tion. (See Fig. 41.) 



116. Insolation received by tlie soil— The sun supplies 

 practically all of the energy by means of which the soil main- 

 tains a temperature suitable for its normal activities. Energy 

 from other sources is negligible. Radiation, the means by 

 which this transfer is affected, is a free wave movement of 

 some type. It is an oscillatory phenomenon, the space between 

 the sun and the receiving body being, so far as is known, en- 

 tirely unaffected. The length^ of such oscillations varies from 



* The approximate wave lengths are as follows : 



Infra-red 000270 to .000075 cm. 



Light waves .000075 to .000036 cm. 



Ultra-violet 000036 to .000019 cm. 



