INFLUENCE OF TEMPERATURE ON LIFE 143 



other purposes. In extreme northern latitudes the low 

 temperature of the soil limits both the kind of crops which 

 can be grown, and the amount of produce obtained. The 

 much greater length of day in summer time does not com- 

 pensate for the deficiency of soil temperature. 



Besides the direct influence of the temperature of the soil 

 on the growth of crops, it has a very considerable influence 

 on the activity of the lower forms of life with which a fertile 

 soil abounds. The dead organic matter of the soil, the 

 remains of vegetable and animal tissue, is made again avail- 

 able as plant food by the successive action of various kinds 

 of fungi and bacteria. The production of nitrates in the 

 soil, a process having a most intimate connexion with its 

 fertility, is, for instance, brought about by the successive 

 action of several species of bacteria. The activity of these 

 living agents is entirely dependent on the temperature of the 

 soil in which they live. At the freezing point their action 

 is practically nil ; it increases as the temperature rises till the 

 optimum temperature is passed, when a rapid decline sets in. 

 Vital action seldom continues beyond 50 C. The soil tem- 

 perature most favourable to the chemical activity of bacteria 

 varies with different species ; it generally lies between 30 

 and 40 C. 



The temperature of the soil also affects the physical pro- 

 cesses occurring in it. We have already noticed (pp. 87, 129) 

 the great influence of temperature on the movements of water 

 and air within the soil. Temperature also affects the move- 

 ments of salts (p. 193). Indeed, there is probably no physical 

 process within the soil which is not affected by temperature. 



The chemical processes within the soil are equally influenced 

 by temperature. The chemical changes in dead matter which 



