6 



Temperatures of soil at a depth of 1 foot (30 centimeters ) fro7n June, 1902, to 

 June, 1903 (degrees centigrade). 



The maximum temperature for the year, 13.2° C. (56° F.), 

 occurred in July, and the minimum, —3° C. (26.6° F.), in De- 

 cember, giving a total annual variation of 16.2° C. (29.4° F.). 



When plants are cultivated in pots, in green houses, the 

 small volume of soil around the roots responds much more 

 rapily to changes in the temperature of the surrounding air 

 and to the influence of streams and sprays of water than does 

 the upper layer of soil in the open. In general the soil in the 

 green house will show a much higher average temperature, 

 which, with the other conditions mentioned, makes necessary 

 special treatment on the part of the gardener. If the natural 

 conditions of water supply by precipitation were complied with 

 the needs of the plant would by no means be met under the 

 altered conditions of temperature. 





Fig. 2. — Tracings of thermographic curves of air, and of soil at a 

 depth of 30 centimeters (1 foot), June 9-30, 1902. The daily periodicity 

 and the direct effect upon the soil of changes in air temperatures are 

 illustrated. Centigrade scale. The temperature of the air is shown by 

 the uppermost of each pair of tracings. 



