SOIL HEAT 235 



of soil heat involves not only the question of conduction 

 through solids but through liquids and gases as well. More- 

 over, transfer resistance, which occurs at the boundary of two 

 substances in contact, has much to do with the rate of trans- 

 mission. In addition, the air and water of the soil are capable 

 of considerable movement which makes conductivity studies 

 extremely difficult due to convection currents. 



The heat conductivity of soil is affected by a number of 

 factors which may or may not lend themselves to field con- 

 trol. Important among these are texture, structure, organic 

 matter, and moisture. The influence of the first is clearly 

 shown by the following comparative data obtained by Bou- 

 youcos, 1 with field soils: 



Table XLIX 



RELATIVE CONDUCTIVITY AS MEASURED BY THE TIME REQUIRED 



FOR A THERMOMETER 7 INCHES FROM THE SOURCE OF HEAT 



TO INDICATE A RISE IN TEMPERATURE 



These results are comparative only in a qualitative way. 

 Quantitative determinations are so beset by error that only 

 few investigators have made any consistent attempt along this 

 line. Patten's results 2 expressed as metric K 3 (the heat con- 



1 Bouyoucos, G. J., An Investigation of Soil Temperature; Mich. Agr. 

 Exp. Sta., Tech. Bui. 17, p. 20, 1913. 



3 Patten, H. E., Heat Transfer in Soils; U. S. Dept. Agr., Bur. Soils, 

 Bui. 59, p. 26-28, 1909. 



3 The conductivity of a substance is measured by the number of gram- 

 calories of heat transmitted in 1 second through a cube with 1 centi- 

 meter edges, when the opposite faces differ in temperature by 1°C. The 

 calories of heat transmitted (H) will be proportional to the area of the 



