CHAPTER X. 

 SOIL TEMPERATURE. 



253. Importance of Soil Temperature. None of the chem- 

 ical, physical or biological changes essential to the devel- 

 opment of plant food in the soil and to the action of roots, 

 can take place in the absence of the energy stored up in 

 the soil and indicated by its temperature. When the tem- 

 perature of the soil falls to 32 F. nearly all the life 

 processes become dormant and for most of the cultivated 

 crops and higher plants these cannot begin until a tem- 

 perature above 40 F. has been reached. All living bodies 

 must have their temperature maintained between certain 

 limits in order to have growth take place. 



254. Soil Temperature at Which Growth Begins. Accord- 

 ing to the observations of Ebermayer growth will not be- 

 gin, with most cultivated crops, until the soil has attained 

 a temperature of 45 to 48 F. and it does not take place 

 most vigorously until after it has reached 68 to 70 F. 

 Neither do the niter germs begin the formation of nitric 

 acid from humus until a temperature above 41 F has been 

 reached and its greatest activity is not attained until the 

 soil temperature has risen to 98 F. 



255. Best Soil Temperature for Germination. There is, 

 for most seeds, a certain range of soil temperature under 

 which germination is most rapid, under which the plants 

 become most vigorous, and which ensures the highest per- 

 centage of plants from the seed. This general truth should 

 never be overlooked in the spring when it is possible to 

 plant in a too cold soil. In the table which follows are 



