236 NATURE AND PROPERTIES OF SOILS 



ductivity coefficient in C.G.S. units) shows the same general 

 comparisons as already presented: 



Table L 



CONDUCTIVITY COEFFICIENTS OF DIFFERENT DRY SOILS 



It is evident, in general, that the finer the texture of the 

 soil, the lower is the conductivity. This cannot be construed 

 as indicating that the conductivity coefficients of sand and 

 clay particles are particularly different. The variance ob- 

 served is adequately explained by the great number of trans- 

 fers necessary in a fine-textured soil. It is also evident that 

 the addition of organic matter will lower conductivity. 

 Humus itself has a low conductivity coefficient and would 

 markedly affect the transfer resistance by changing the struc- 

 ture of the soil. Compacting a soil should accelerate heat 

 transfer due to a more intimate contact of the soil grains and 

 a consequent diminution of transfer interference. Tillage, 

 on the contrary, must impede not only the movement of heat 

 downward in the soil but from the subsoil into the furrow 

 slice. 



The greatest single factor to be considered in heat conduc- 

 tivity is the moisture content of the soil. The curve (Fig. 44) 



faces (A) and to the differences in temperature of the faces (f — t"), 

 while it will be inversely proportional to the thickness (d) of the cube. 

 K is a constant, depending on the material studied. 



H=;K .A(t--t"), 

 d 



