34 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



applied to all the hours of the day which it represents. This impli< 

 of course, that the range of the instrument has been adjusted before 

 it is placed in service. 



Summary of Soil Temperatures. 



In addition to the means by decades and months, the "Soil Tem- 

 peratures " form may be shown the number of days, for each depth at 

 which readings are taken, with temperatures below :V2° F. (frozen); 

 with temperatures 32.1 to 41.0° (cold); with temperatures 11.1 to 

 50.0° (cool); with temperatures 50.1 to 60.0° (warm): and with 

 temperatures above 60° (hot). 



Annual Summaries of Soil Temperatuki s. 



The ^Summary" formnuvy be used for a summary <>f one or several 

 soil temperature conditions, such as the mean temperatures by 

 decades, months, growing seasons, and years, or the number «>f days 

 of each temperature class in each month. In the case of surface 

 temperatures, the mean and absolute maxima and the daily pang 

 are doubtless of great interest. As many forms a- necessary may he 

 used. 



It may be found that a given soil temperature sufficiently delimits 

 growth so that the occurrence of such a temperature marks the he- 

 ginning and end of the growing season. This has been the idea in 

 suggesting a division of temperature computation- at M I\ or 5°C, 

 such a temperature being approximately the minimum for activity of 

 lower forms of plant life, as shown by numerou- experiment-. 



Apparatus. 



The most simple apparatus for measuring soil temperature- i- the 

 encased soil thermometer, having a stem of sufficienl length so that 

 the mercury appears above the surface of the ground when the bulb 

 is at the desired depth. As ordinarily made, however, this thermom- 

 eter is not only very expensive but is inadequately protected from 

 exposure to the elements and to mechanical forces. For this reason 

 it is not desirable for permanent stations, but will probably in many 

 cases be useful where observations are temporary and light equip- 

 ment is desired. 



For permanent stations the most serviceable apparatus that has 

 been thoroughly tried is an ordinary thermometer suspended by ;i 

 cord in a 1-inch iron pipe, whose lower end may be sealed either by 

 a cap or by welding. The latter is preferable where the pipe must 

 be sunk to any -great depth, since the welded pipe may he formed 

 as a wedge and may be driven into position without seriously dis- 

 turbing the soil. The pipe should, in all eases, he long enough to 

 extend well above the ground and above any ordinary >n<»\\ cover- 



1 



