282 AZOF I CAT I ON 



over from time to time. Lohnis observed that Azotobactcr mem- 

 branes are more readily obtained in winter than in summer. He 

 later found that the nitrogen-fixing power of soil varies from month 

 to month throughout the year, there being two maxima one in 

 spring and another in autumn. The extent of the variation noted 

 may be seen from the following: 



1903-1904: March 100 



May 121 



July 50 



September 100 



1907: April 100 



" May-June 133 



" July-August 69 



" October-November 122 



The relative numbers are based on the spring months as 100. 



Green found nitrogen-fixation in 1 per cent, mannite solution to be 

 low during August, September and April. In other months he 

 noted a fairly constant fixation of about 10 mgm. of nitrogen per 

 gram of mannite. He also noted a marked yearly variation in the 

 nitrogen fixed during July and August. 



Walton found nitrogen-fixation lowest in Indian soil between 

 October and January and highest between June and September. 

 This corresponds with moisture and temperature changes. Peterson 

 has found that although the nitrogen-fixation of Utah soils is highest 

 from June to September, the number of types of Azotobacter occur- 

 ring in the soil was greatest in May. Moll goes so far as to maintain 

 from his work that the season of the year is the principal factor in 

 determining the biochemical transformation in soils. This would 

 appear to be especially true as regards nitrogen-fixation. 



Crop. Heinze called attention to the fact that the fallowing of the 

 soil increased its nitrogen-fixing power. This could be due to better 

 aeration, moisture, temperature, etc., and not to any depressing 

 influence exerted directly by the plant, Most experiments which 

 deal with plant and bacterial activity could be interpreted in this 

 light. Hiltner maintained that the free nitrogen-fixing bacteria 

 are stimulated in their activities by the growing plant roots. There 

 may be considerable truth in this, for here the higher plants are 

 rapidly removing from the solution the soluble nitrogen compounds. 

 In this case, the nitrogen-fixing organisms would be forced either to 

 compete with the higher plant for the soil nitrogen or else to make 

 use of their ability to live upon the atmospheric nitrogen. It is 

 certain that different cultural methods vary sufficiently with crops 

 to influence profoundly a soil's nitrogen-assimilating properties, 

 for the Azotobacter occur more widely distributed in cultivated than 

 in virgin soil. The analyses of hundreds of samples of cultivated 

 and virgin soils in Utah have in nearly every case shown the virgin 

 soil to have a low nitrogen-fixing power as compared with the culti- 



