DAMPTXC.-OFF IN FOREST NURSERIES. 



75 



TABLE XI. Result* of inocululinn, at tlir time of xoirhifi, irith Pythiu-ni 

 ilctHiri/itnuni on I'i-tiH* iHiiikxitinn ir different s<nri)tii (Jcuxitics in pots of 

 autocld r<<! xo?7. 



[The percentages of " Damping-otT," columns 4 and 7 . are based on the number of seedlings; the percentages 

 given in columns 3, 5,6, and *are based on the number of seeds.] 



MOISTURE AND TEMPERATURE FACTORS. 



The relation of clamping-off to moisture and temperature are sub- 

 jects less easily studied. In 190T and 1908 Mr. W. H. Mast, then 

 supervisor of the Nebraska National Forest, conducted daily .counts 

 of the number, of seedlings damped-off and compared these records 

 with temperature and rainfall records. The writer in 1909 repeated 

 his work, maintaining parallel records of damping-off, air and soil 

 temperatures, soil moisture, atmospheric humidity, wind movements, 

 and evaporation. The 1909 records of damping-off, temperature, 

 soil moisture, and evaporation appear in figure 20. The damped-off 

 seedlings were counted and removed in the morning and evening, the 

 day period thus being in most cases 10 to 11 hours and the night 

 period 13 to 14 hours. Because the period was not always the same 

 length, the data are reduced to a per hour basis. Air temperature 

 was recorded by a sheltered thermograph 3 feet above the soil sur- 

 face. The evaporation graph represents the mean loss per hour 

 from two porous cup atometers of the writer's own design, in which 

 the rather long and slender Chamberlain filter bougie was used and 

 supported in a horizontal position just above the soil surface so as 

 to be under as nearly as possible the same atmospheric conditions as 

 the seedlings. The two bougies were placed at right angles to each 

 other in. order to eliminate as far as possible the effect of change of 

 wind direction on their mean los*>. While the rain-correction mount- 

 ing had not "at that time come into use, the error due to rain absorp- 

 tion appeared negligible; atometers filled shortly before rainfall 

 were read immediately after without any gain being found in the 

 water in the reservoir. The psychrograph and wind-movement rec- 

 ords are not presented, as the evaporation values are more easily inter- 

 preted. Soil moisture was periodically determined in the soil of the 



