Studies in Soil Physics, V. 187 



serves a little nearer attention but as a preliminary thereto it 

 will be well to review briefly the means by which heat is added 

 to or abstracted from the soil 



The ultimate source of nearly all soil heat is the radiant 

 energy of the sun, and indeed this is the source of much of it 

 immediately as well as ultimately. Direct or reflected sunlight 

 is the great soil warmer. The soil also loses heat b\' radiation, 

 mainly during the ni,';ht, but the amount is usually less than 

 that gained similarly during the day. Next to radiation the 

 largest additions or subtractions of heat are by means of the 

 entering or leaving water. If the soil is cooler or warmer 

 than the rain which falls on it this rain will warm or cool it and 

 indeed this is frequently the most practical factor aff'ecting the 

 soil temperature. It ma\ require days of sunshine to counter- 

 act the cooling effect of a few hours of cold rain. Of course the 

 amount of heat "added" or "subtracted" by rain falling on 

 the soil is measured not simply by the temperature of the rain 

 but by the amount and sign of the difference in temperature 

 between the water entering the soil and the same water as it 

 leaves in the drainage. If the water leaves cooler than it en- 

 tered it has warmed the soil, and conversely. 



Besides radiation and added water there is only one major 

 means of heat transfer-the loss of heat occasioned by the evap- 

 oration of water from the soil surface. Even this is only oc- 

 casionally of practical importance and the previously named 

 factors remain the only ones which need \crv careful consider- 

 ation. There are also many still less significant ways in which 

 heat is lost or gained, among which may be enumerated: (1) the 

 heat deri\ed from or given up to the air in contact with the soil, 



(2) the heat transferred by conduction to or from lower strata; 



(3) the heat derived from radioactive materials in the soil; and 



(4) the heat given up by ])lants * and animals to the soil. All 

 these are very small in amount and entirely unimportant. 

 From our present viewpoint it is also unnecessary to concern 

 ourselves with the mechanism of the transfer of heat from place 

 to place in the soil, especially as this matter has already been 



•Contrary' to an opinion still sometimes expressed the soil does not give up heat to 

 the plant Quite the reverse Under usu;il growing conditions the plant is nearly al- 

 ways warmer than the soil and heat flows from root to soil instead of in the reverse 

 direction. The plant nui'-t be kept in warm soil not because heat, as such, is necessary 

 to its growth but because it must be maintained at the temperature at which the 

 rrzymolic reactions upon which growth depends are able to go on. If the soil he too cold 

 the plant looses heat k) r.ipjdly lh.it (his optiiiiuni temperature cannpt be maintained 



