F. HEAT OF THE SOIL 



I. FUNCTION OF THE HEAT OF THE SOIL IN ITS 

 RELATION TO PLANT GROWTH 



The heat of the soil has three general functions with 

 reference to plant growth. These are: (1) Biological. 

 (2) Chemical. (3) Physical. 



315. Biological. Heat is the motive power in plant 

 growth. A certain degree of heat is necessary for the 

 normal action of all of the functions of the plant. When 

 the soil, as well as the atmospheric temperature, passes 

 beyond a certain maximum or minimum degree, growth 

 is inhibited. These points differ for different species 

 and groups of plants, and they may be different for 

 different individuals of the same species. Somewhere 

 between the maximum and the minimum temperature 

 which any plant can withstand and still live, is the 

 optimum or best temperature for growth. These rela- 

 tions may be divided into the following three groups. The 

 best soil temperature for: (1) Germination. (2) Growth 

 and vegetation. (3) Proper activity of the soil organ- 

 isms. 



316. Germination. This takes place at widely dif- 

 ferent temperatures for different plants. Ordinarily, 

 the optimum temperature for germination is several 

 degrees below the optimum temperature for growth 

 during the average period of vegetation. 



The range for a few common plants is shown in the 

 following table: 



(448) 



