234 NATURE AND PROPERTIES OF SOILS 



soil of low volume weight may warm up easily when dry, its 

 high water content usually markedly retards its temperature 

 change. A muck soil is usually the last to freeze in winter 

 and, conversely, the last to thaw in spring. The advantage 

 of drainage is evident as a wet soil is of necessity colder in 

 the spring than one that is well drained. This at least par- 

 tially accounts for the fact that a sandy soil is usually an early 

 one and is, therefore, of particular value in trucking. 



122. Heat movements in soil. — While volume weight, or- 

 ganic matter, and moisture seem largely to control the degree 

 to which a soil will become heated when exposed to insolation, * 

 it is evident that there must be some mode of energy transfer 

 whereby such phenomena may be facilitated. Heat movement 

 is necessary in order that the lower layers of the soil may 

 become warm enough for proper biological functionings. 

 Energy transmission both downward and laterally is abso- 

 lutely essential and deserves as much attention as the factors 

 influencing insolation absorption. 



Two methods of heat transfer function in a normal soil — 

 conduction and convection. These modes of energy move- 

 ment are extremely difficult to analyze, due to the impossi- 

 bility of controlling one while studying the other. 



123. Conduction of heat in soil. — While radiation has to 

 do with the oscillatory transfer of energy conduction relates to 

 the molecular transmission of heat through any material. 

 When one part of a substance is heated, the movement of its 

 molecules is stimulated. These molecules strike their neighbors 

 with increased force, thus quickening their motion. These in 

 turn accelerate others until the energy applied at one point 

 becomes apparent at another. Solids as a class are better con- 

 ductors than liquids, while liquids in general are superior to 

 gases in this respect. It must be remembered in studying the 

 conductivity of heat through soil, that we are dealing with 

 a heterogeneous mixture of mineral and organic matter con- 

 taining varying amounts of air and water. The movement 



