186 I'he Pi,ant W0R1.D. 



to recently *, and in which the abnormality was so great that, 

 facetiously, the name Taraxacum paradoxa was suggested for 

 the "new species." 

 University of Michigan. 



Ann Arbor, Michigan. s 



STUDIES IN SOIL PHYSICS, Y. i 



Soil Temperature. 



E. E. Free. 



The close dependence of plant growth upon the existence 

 of a proper surrounding temperature is a commonplace of plant 

 physiology and as a part of the ''surrounding temperature," 

 the temperature of the soil is scarcely less important than that 

 of the air. Furthermore, germination is even more closely 

 controlled by temperature than is growth, and for germination 

 the soil is the sole environment and its temperature the only 

 one effective. Among the soil factors to which the physiolo- 

 gist and ecologist must give attention temperature is by no 

 means the least. Nor is it legitin ate to assume, as is often done, 

 that the soil temperature is sufficiently indicated by the average 

 temj:erature of the air. There is a relation, of course, but not 

 a direct nor exclusive one. Soil temperature is not a static 

 thing but results from a balance between the quantities of 

 heat being added to the soil and being taken fr( m it. When 

 more heat is entering than is lea\ing the soil grows warmer; 

 when the reverse condition obtains it grows cooler. Its tem- 

 perature is seldom constant, being subject not only to the usual 

 diurnal and annual c\cles,lnit also to many accidental changes. 

 The wider of these variations are climatic matters, under the 

 the control of meteorological factors v. ith which the specific 

 nature of the soil has little or nothing to do. Such are outside 

 our present interest. The soil character does, however, influ- 

 ence its temperature, and sufficiently importantly to have some 

 phvsiological and ecological significance. This matter de- 



♦American Botanist 15: 27. 1909. 



t Published by permission of the Secretary of Agriculture. 



