80IL HEAT 



313 



ent for quartz powder as drawn by Patten ^ (see Figs. 50 

 and 51) illustrate the effect of distance on temperature 

 rise, the conductivity coefficient remaining constant. 



Decrees C. 



so TJmemmirf* 



Fig. 50. — 'Temperature time curves for quartz powder at various dis- 

 tances from the source of heat. 



From this brief discussion of conductivity it may be 

 established that such a movement is of importance to 

 plants in carrying heat downward into the soil. While 

 it is affected directly by tex- 

 ture, structure, and humus 

 to a certain extent, moisture 

 is the dominant factor. Under 

 natural conditions it is neces- 

 sary to maintain a medium 

 moisture content, although 

 the conductivity of heat is 

 not then at its maximum. 

 However, it must always be 

 remembered that convection 

 is active under such conditions and may do much in 

 facilitating heat distribution. Good tilth and increased 

 organic content of any soil, by raising the optimum 



1 Patten, H. E. Heat Transfer in Soils, U. S, D. A., 

 Bur. Soils, Bui. 59, pp. 23-24, 1909. 



7cm 



r z 3 4. s ^^Jc^ 



Fig. 51. — Temperature gradi- 

 ent for air-dry coarse quartz 

 powder. 



