68 



Journal of Agricultural Research 



Vol. xvir, No. 2 



stant correlation with air temperatures, and it is highly probable that 

 a fairly constant ratio may be calculated which will enable the exten- 

 sive data on air temperatures to 

 be used in considering relations of 

 soil temperatures to disease. 



The data taken in connection 

 with the studies presented in this 

 paper were started in the spring of 

 1 91 5. For this purpose electrical 

 resistance thermometers were used. 

 These were buried in the soil in 

 tobacco fields at the Station farm 

 at depths of 2, 4, and 8 inches. 

 Some of the thermometers were 

 buried in such a way that they 

 would record the temperature of 

 soil becoming gradually shaded by 

 the growing tobacco, while with 

 others the soil was exposed con- 

 tinuously to the full sunshine. 

 The latter temperatures are the 

 ones upon which conclusions were 

 drawn, since in a badly infested 

 field shading of soil would be rela- 

 tively small, owing to the poor 

 growth of the crop. On the other 

 hand, where very heavy infestation 

 does not occur or a relatively re- 

 sistant variety is used, the relative 

 importance of shading must be 

 considered (fig. i). 



The temperature readings were 

 taken with duplicate thermometers 

 each day at i p. m. This hour 

 was selected as it was the most con- 

 venient time of the day to take 

 the readings. No great im-por- 

 tance, however, can be attached to 

 the time of taking daily readings, 

 on account of the great daily varia- 

 tion which occurs especially near 

 the surface of the soil. It would 



be most desirable to record the minimum and maximum tempera- 

 ture for each day, but in using electrical thermometers this would 



