4 N. H. AGE. EXPERIMENT STATION. [Bulletin 12 



The growth curve follows the air temperature more closely, how- 

 ever, than any other external factor recorded. There is no close 

 correlation between the humidity curve and growth curve. It 

 is not possible to control the separate factors under field condi- 

 tions. These conclusions are based on 43,000 measurements dur- 

 ing three seasons. 



4. The growth is much more readily affected by external 

 factors in the early period of its growth than when it is approaching 

 the resting period. 



5. The grand period of growth in this orchard was a period of 

 about 25 days (3 years considered). 



SOIL TEMPERATURE. 



Among the external factors that influence the growth of our 

 trees, the soil temperature doubtless plays a part. Hence we may 

 inquire what the differences in temperature would be between 

 plots treated in various ways in the orchard and whether they are 

 very marked in character. Since bacteria and soil flora have 

 certain optima of temperature at which they best thrive and 

 since these agencies in turn are responsible in part for growth the 

 question at issue assumes some importance. 



Although it has not been possible to make extensive observa- 

 tions of the soil temperature factor, yet by the use of accurate in- 

 struments a number of readings were made which give us some 

 index of the temperature conditions which obtain under the 

 various soil treatments. The apparatus used was a set of Resist- 

 ance Thermometers, manufactured by Leeds and Northrup Co., 

 Philadelphia, Pa. A bulb was placed eight inches below the 

 surface of the soil in Plots 1, 4, 5, 7 and 9; and one at three feet 

 below the surface in Plots 1, 4 and 7, in close proximity to the ones 

 near the surface. 



The readings were made each day at 2 o'clock p. m. except 

 Sundays, and an occasional day in addition. While we do not 

 know what the maximum temperature was we assume that the 

 readings at 2 o'clock would not be far from giving the maximum 

 for that day. 



From the investigations reported in Tech. Bulletin No. 26, 

 Michigan Agricultural Experiment Station, it is seen that a study 

 of the temperature of soils is a complicated matter. The soil 



