BOOKS AND CURRENT LITERATURE 



Factors Influencing Nitrogen Fixation. — From the Virginia 

 Agricultural Experiment Station has recently appeared another valu- 

 able article in the series of studies of the bacterial flora of the soil.' 

 The object of the experiments reported in this paper was to determine 

 the importance of various factors that influence the nitrogen fixing 

 power of soil bacteria. Among the factors mentioned are soil moisture, 

 humus content, toxic materials, and effect of salts. 



In determining the influence of moisture content, soils which had 

 been tested for nitrogen fixing power, and from which Azotohacter had 

 been isolated, were stored in glass jars for twelve to eighteen months 

 when they were practically air dry. Qualitative examinations were 

 then made with the result that Azotohacter was found in only four of 

 twenty-five soils from which it had previously been isolated. The 

 moisture content was now increased to 20% and the soils incubated 

 at 28° for three weeks, when tests showed that Azotohacter was present 

 in only three of a number of soils in which it had previously been 

 found. The evidence points to a deterioration of Azotohacter as a 

 result of drying. In another set of experiments, soils stored in the 

 laboratory for fifteen months lost 24 to 43% of their original nitrogen 

 fixing power, and it is suggested that in the light of the experiments 

 previously mentioned this represents in a large measure the loss due 

 to the lessened vitality of Azotohacter. The effect of drying, as shown 

 by the decreased power to fix nitrogen, became evident in some soils 

 in ten days to two weeks, yet after fifteen months other soils showed 

 considerable nitrogen fixing power. This suggests that some nitrogen 

 bacteria are much less affected than Azotohacter by decrease in the 

 moisture content of the soil. Toxic substances, which have been 

 regarded by some plant physiologists as the cause of much if not all 

 soil sterihty, evidently have but little influence on nitrogen fixing 

 bacteria, for cultures in extracts of sterile soils and control cultures 

 were about equally efficient. Addition of Kme to soils of optimum 

 water content and fair nitrifying power stimulated nitrogen bacteria 



1 Williams, Bruce. Some Factors Influencing Nitrogen Fixation and Nitri- 

 fication. Bot. Gaz. 62: 311-317. 1916. 



379 



