SOIL HEAT 293 



in around the earth heat that would otherwise be entirely 

 lost so far as the soil is concerned. 



(3) A certain amount of heat may be brought to the 

 soil by precipitation. A warm spring rain, by falling 

 on the earth and percolating into its subsoil, may be a 

 determining factor in crop growth. Although the 

 aggregate amount of heat added in this way is small, 

 the opportuneness of its application is of no small 

 importance. A warm rain often imparts an impetus- 

 to plant growth which may be noticeable for many weeks 

 afterward. 



(4) A large amount of heat is annually entrapped by 

 growing plants. This energy is stored up and may ulti- 

 mately be liberated by the decay of the tissue. If such 

 oxidation takes place in the soil, as it very largely should 

 under good farm management, a certain amount of heat 

 is liberated in the soil. How important this is it is 

 difficult to say, for such energy is given off so grad- 

 ually as to be rendered difficult of measurement. Bac- 

 terial activity is very closely allied to the utilization 

 of such heat. Except under exceptional conditions, as 

 in hotbeds or very heavily manured lands, such heat 

 has no appreciable effect in altering the temperature of 

 a normal soil. 



210. Factors affecting soil temperature. — The tem- 

 perature that the soil of any given locality may attain 

 is dependent on a certain group of factors so closely re- 

 lated as to make their separate consideration sometimes 

 rather difficult. For convenience these factors may be 

 listed as follows, the actual temperatures and their 

 probable fluctuations under field conditions being re- 

 served until the various intrinsic and external factors of' 

 soil heat have been discussed : — 



