THE EVALUATION OF THE SOIL TEMPERATURE 

 FACTOR IN ROOT GROWTH 



W. A. CANNON 



Desert Laboratory, Tucson, Arizona 



When reduced to its lowest terms the information necessary 

 for use in evaluating the temperature factor in the growth of 

 roots includes in addition to the temperature itself, the rate of 

 growth at different soil temperatures^ With such data, and by 

 employing the time factor, we are in position to calculate the 

 TR^ for any species. This in brief is the total expected root 

 growth of the species, that is to say, the suimnation of root 

 growth at the temperatures employed. 



This method is of some interest from an ecological as well as 

 from a physiological point of view. AVhere a species is restricted 

 in its general distribution by unfavorable temperatures, it is 

 probable that at such limits of distribution the value of TR 

 would be relatively small, whereas at the center of the range it 

 would be relatively large. It therefore seems possible, were 

 sufficient data on hand for the purpose, to plot the TR as iso- 

 lines. The broad effects of different soil temperatures on root 

 growth, and the relation of the root-soil temperature response 

 to the general distribution of the species could in this manner 

 be clearly brought out. 



This physiological method of temperature evaluation as a 

 factor in root growth can be sufficiently illustrated by a single 

 example. For this purpose reference can be made to the rates 

 of root growth of Covillea tridentata, and these can be related to 

 soil temperature at a depth of 30 cm. at two stations, namely, 

 the Coastal Laboratory, and the Desert Laboratory, for the 

 month of August, 1916. It will be recalled that Covillea is one 



1 Rate of hourly root growth for any species at any given soil temperature 

 multiplied by the number of hours the soil was at that temperature. 



64 



