SOIL HEAT 



325 



FIG. 54. Curves showing the hourly temperatures of bare soil at a 

 depth of four inches and of the air above the soil. , May 26, Germany. 



The temperature of the soil at the surface may often 

 exceed that of the air, and the amount of daily fluctua- 

 tion may be greater ; but for the lower depths the tem- 

 perature curve flattens out. The subsoil shows but 

 little daily, and even monthly, variation, and is affected 

 only by seasonal changes. 



228. Control of soil temperature. The means of 

 practical control and modification of soil temperature 

 are those commonly in vogue in good soil management. 

 'The most important factor is, of course, soil moisture. 

 Good drainage, proper tilth developed by deep plowing, 

 plenty of lime, and sufficient organic matter, favor opti- 

 mum moisture conditions. Such moisture regulation 

 means a lowered specific heat and good conductivity. 

 The use of a soil mulch or an artificial covering not only 

 will check evaporation but will markedly retard loss of 

 heat by radiation. Anv farmer who so controls his soil 



