688 STATE BOARD OF AGRICULTURE. 



SOIL TEMPERATURE 



Technical Bulletin No. 2G. 



BY GEORGE J. BOUYOUCOS. 



INTRODUCTION. 



lu the fall of 1911 an investigation of the general subject of soil tem- 

 perature was undertaken at this Station. Although the subject had al- 

 ready received a large amount of attention yet it was not considered 

 solved, as our knowledge concerning it appeared very unsatisfactory. 

 This is undoubtedly due to the fact that the problem is a most complex 

 one, as the number of factors influencing it is very great. These factors 

 may be divided into two general groups, (1) the intrinsic and (2) the 

 external. The intrinsic factors are those contained by the soil and com- 

 prise the specific heat, heat conductivity, thermal absorption and radia- 

 tion, specific gravity, texture, structure, moisture content, salt content, 

 topographic position, nature of surface, etc. The external factors in- 

 clude the meteorological elements, chief of which are the air temperature, 

 sunshine, wind velocity, barometric pressure, precipitation, dew point, 

 humidity, etc. The intrinsic factors vary with the different kinds of 

 soil and tend to exert a temporary or permanent controlling influence 

 upon the soil temperature. They are acted upon by the external factors 

 which cause the soil temperature to fluctuate as they themselves vary. 

 Inequality in temperature, therefore, in different kinds of soil for any 

 particular day will depend upon their intrinsic factors, but variation in 

 temperature in these same soils for succeeding days or seasons will de- 

 pend upon the external factors. Both of these groups of factors may be 

 subdivided into two parts, one part tending to take up or import heat to 

 the soil system and thereby raise its temperature, and the other part 

 tending to give up or take away heat from the same soil system and 

 thereby lower its temperature. These opposing factors are in operation 

 practically all the time. The temperature of a soil, therefore, under 

 field conditions, is the resultant of all these variables and contesting 

 forces. 



Most of the previous investigators usually studied only one or two of 

 the intrinsic factors influencing or controlling the soil temperature and 

 then, as a very general rule, deducted general conclusions therefrom as 

 to the importance and controlling effect of these factors upon the soil 

 temperature. There appears to be no one investigator who attacked the 

 subject thoroughly and systematically from all standpoints. As a result 

 wrong conclusions have been drawn, and are being disseminated at pres- 

 ent, as to the importance and controlling effect of some of the intrinsic 

 factors upon soil temperature, which have led to erroneous beliefs con- 

 cerning the difference in temperature in the dilferent classes of soil. 



