238 NATURE AND PROPERTIES OF SOILS 



resistance. Heat passes from soil to water about 150 times 

 easier than from soil to air. As the water increases, the air 

 decreases and the rate of conductivity is raised. When suf- 

 ficient water is present to join all of the soil particles, further 

 additions will have little effect on character of heat movement. 

 Moisture, optimum for crop growth, amply provides for heat 

 transfer. The slow warming up of the lower subsoil must not 

 be taken as an indication of lower conductivity. It is due 

 rather to a lessened heat supply. As a matter of fact, the 

 rate of heat transmission has been shown to be more rapid in 

 the subsoil, due to a greater compaction and to the presence 

 of more water. 



This brief discussion of conductivity shows the vital im- 

 portance of such a phenomenon to plants in that the necessary 

 heat is carried broadcast through the soil. While conduc- 

 tivity is affected to a certain extent by texture, structure, and 

 organic matter, moisture is the dominant factor. Under nat- 

 ural conditions, it is necessary to maintain a medium amount 

 of water in the soil. This moisture condition, fortunately, 

 supports almost maximum heat conduction. Good tilth and 

 increased organic matter probably exert their greatest in- 

 fluence on this type of heat transfer by their influence on soil 

 moisture. 



124. Convection transfer of heat. — Convection, the third 

 manner by which energy may be conveyed, is a heat transfer 

 by means of currents in liquids or gases. It functions by an 

 actual and obvious movement of matter. In the soil absorp- 

 tion tends to heat the air as well as the solid substance. This 

 produces currents due to the expansion and rise of the warmed 

 gases. It is obvious that such heat movement must always be 

 lateral or upward, never downward. Such convection exerts 

 its greatest influence in equalizing the temperature of the soil, 

 overcoming the effects of unequal conduction and uneven ab- 

 sorption due to vegetation or stone. Air currents as they 

 escape into the upper air carry considerable heat away from 



