306 



SOIL PHYSICS AND MANAGEMENT 



8. Conductivity of Soil Material and Soils. Wet soils are 

 better conductors of heat than dry ones and compact ones better 

 than loose ones. These differences are due to the fact that air is a 

 very poor conductor, even poorer than water. Soils should not con- 

 duct heat downward very rapidly in spring, but should cause concen- 

 tration of heat in the surface two to four inches to hasten germina- 

 tion and aid the growth of the 'young plant. Of all soil materials 

 quartz shows the highest rate of conductivity, while dry powdered 

 chalk shows the lowest. 



In the following table it will be well to note the difference 

 between loose and compast, wet and dry, and fine and coarse. 



Relative Conductivity of Soil Material 13 



Soil material 



Quartz powder 



Peat 



Kaolin 



Chalk.. 



Clay with limestone stones 



Clay with quartz stones 



Quartz sand, fine 



Quartz sand, medium 



Quartz sand, coarse 



Quartz sand 



The next table shows the length of time required after the air 

 temperature had begun to rise for the heat to penetrate the soil to 

 the depths given in the table. The conductivity of soils does not 

 play a great part in practical agriculture except early in the spring 

 when the greater conductivity of sand soils permits them to warm 

 up earlier and to a greater depth, thus giving the crops grown upon 

 them the advantage of several hours of warmer soils each day. 



Relative Time for Heat to Penetrate the Soil Under Field Conditions 14 



