490 STATE BOARD OF AGRICULTURE. 



FORE WOK D. 



Technical Bulletin No. 17. 



BY GKOUGE J. liOUYOUCOS. 



For nearly two years Dr. Geo. J. Bonyoucos has been making a study 

 of some of the factors affecting the temperature of soils. A description 

 of some of his work and the conclusions drawn therefrom form the sub- 

 ject matter of this bulletin. 



This work has been deemed important for the reason that some of 

 the conclusions usually accepted concerning soil temperature have not 

 been satisfactory — have not seemed to accord with observed facts. Ap- 

 parently these conclusions have been drawn, in part at least, from 

 analogy, and in part without giving due weight to other physical fac- 

 tors involved. Working with facts rather than from analogy, and giving 

 to various factors their proper values the general problem becomes 

 clearer and the conclusions are in accord with the facts of soil behavior. 



JOS. A. JEFFERY, 



Soil Physicist. 



INTRODUCTION. 



Soil temperature is one of the essential limiting factors of plant 

 growth. It affects the three most important functions in the soil: the 

 biological, the chemical, and the physical, and thereby controls to a 

 very large extent, the productive power of a soil. 



The biological effects at once become evident when it is considered 

 that all life depends not only for its progress, but also for its existence 

 upon soil temperature. Indeed, the germination of seed, the maximum 

 growth of plants, the multiplication and functions of the lower organ- 

 isms, the taking up of food by plants through osmotic processes, etc., 

 depend to a very large extent, upon soil temperature. Moreover, all 

 these biological processes attain their maximum end not at the mini- 

 mum, nor yet at the maximum, but at the optimum temperature, which 

 oi)timum temperature is diUcrent for the various kinds of life. 



To preceive the chemical role that temperature plays in the soil it 

 will suflice merely to mention that it is a great accelerator of all 

 chemicnl reactions: it iiiflnonccs the solvent action of water; it influ- 

 ences the velocity of the reaction ; it increases the osmotic pressure of 

 the solution; it favors the formation of nitrates; it hastens the weather- 

 ing of mineral material and the decomposition of organic matter. 



