1920-21.] Relation of Soil Colloids to Conductivity of Soil. 67 
which contain organic matter or soil colloids — compared with the larger 
It 
fluctuations of =p- 3 in clay loam, which is rich in colloids, is a point of great 
significance in the present investigation. 
IV. Conclusions. 
The addition of organic matter to a soil reduces the conductivity of that 
soil. Thus Bouyoncos found that sand with the addition of 332 per cent, 
organic matter was a better conductor than sand with 6'95 per cent, organic 
matter ; moreover, in the soils he tested, the conductivity, both dry and wet, 
was in the inverse ratio to the organic matter present.* 
Therefore the change in conductivity of clay loam cannot be due to the 
organic matter present, but must be caused by the colloids present in that 
soil. This change of conductivity is probably brought about by the 
colloidal films surrounding the soil grains swelling with the rise in 
temperature, and so automatically compacting the soil, and reducing the 
transfer resistance to heat between the particles within the soil. 
* “ An Investigation on Soil Temperature and some of the most important factors 
influencing it,” G. J. Bouyoncos, Technical Bulletin No. 17, 1913, Michigan Experimental 
Station . 
{ Issued separately May 9, 1921.) 
