SEASON 28l 



heated just above the temperature which Cunningham and Lohnis 

 found to be the thermal death-point of soil protozoa. 



Light and other Rays. As a class, bacteria are sensitive to light, 

 but the extent to which they can withstand it varies, among other 

 things, with the conditions of exposure and the specific organism. 

 Unfortunately, we have but fragmentary information concerning the 

 effect of light upon azofiers, but what we do know would lead us to 

 believe they are more resistant than many microorganisms prob- 

 ably more so than many other soil bacteria. Berthelot recognized 

 that nitrogen-fixation in the soil occurred both in daylight and in 

 darkness, though more freely in the light. Jones found many 

 Azotobacter to be alive in a small Petri dish of dried soil that had 

 stood in the laboratory in front of a south window for two years. 

 They can withstand the direct action of the violet and of the longer 

 ultraviolet rays for five minutes, but are killed in much less time by 

 the shorter ultraviolet rays. They are more resistant even to these 

 than are many other species. 



The fixation of elementary nitrogen by A. chroococcum is distinctly 

 increased when the air is activated by pitchblende. Somewhat 

 better results are obtained with weak than with stronger radio- 

 active intensity. 



Aeration. Under field conditions there is a mixed flora consisting 

 of the anaerobic and aerobic nitrogen-fixing microorganisms. A 

 soil condition which would be ideal for one species might be unfavor- 

 able for the other. It has already been pointed out that there are 

 two maxima of nitrogen-fixation in soils, depending upon the 

 moisture content. This is illustrated in Figure 32. 



'Although it is usually conceded that nitrogen-fixation is most 

 rapid when soils are well aerated, this may not always be the case. > 

 Concerning this Murray reports the following results: 



Nitrogen fixed 



Kind of soil. Aerobic Anaerobic 



conditions. condition, 



mgm. mgm. 



Greenhouse soil 0.84 8.50 



Loam soil 3.08 5.29 



Clay soil 0.84 4.69 



* This condition must be attributed to a great difference in the 

 physiological efficiency of the two groups found in these particular 

 soils and not to a lack of aerobic nitrogen-fixing organisms, for more 

 than ten times the number of organisms developed on nitrogen- 

 poor media from these soils under aerobic as under anaerobic condi- 

 tions. > 



Season. Berthelot was unable to show any gain in nitrogen of 

 his soils during the winter, but Koch found a considerable increase 

 during this season in soils which were kept in a heap and shovelled 



