276 AZOF1 CATION 



fixing organisms takes place in the upper soil strata is that of sym- 

 biosis between the aerobic and anaerobic nitrogen fixers, principally 

 between Azotobacter and Clostridium pasteurianum. In spite of the 

 opposite properties of the two species, their synergetic activity in 

 the upper strata of the soil results in a harmonious mutual develop- 

 ment producing the maximum economy in consumption of energetic 

 substances." 



So far, little has been done to determine the relationship of 

 Azotobacter to the higher plants, but it is interesting to note that 

 Beijerinck has observed a distinct relationship between the distribu- 

 tion of the organism and leguminous plants. Fischer suggests that 

 some nitrogen-fixing bacteria presumably exist first as saprophytes, 

 then as exoparasites in loose combination with green plants, then as 

 endoparasites. Finally they develop the true symbiosis or root 

 nodule bacilli. Hopkins has questioned whether there may not be a 

 relationship between the legume bacteria and Azotobacter. 



The Influence of Water. ^Azotobacter are very resistant to drying; 

 they may be dried for a considerable time in a desiccator over sul- 

 phuric acid. Pure cultures are just as resistant to drying as are mixed 

 cultures. ) This would vary some with the media in which the 

 bacteria are dried, for the survival of non-sporebearing bacteria in 

 air-dry soil is due, in part, to the retention by the soil of moisture 

 in the hygroscopic form. This, however, is not the only factor, for 

 the longevity of bacteria in a solid is not directly proportional to its 

 grain size and hygroscopic moisture. < Giltner and Langworth found 

 that bacteria resisted desiccation longer in a rich clay loam than in 

 sand. Furthermore, if bacteria are suspended in the extract from 

 a rich clay loam before being subjected to desiccation in sand, they 

 live longer than if subjected to dessiccation after suspension in a 

 physiological salt solution. Because of this, they consider that soils 

 contain substances which have a protective influence upon bacteria, 

 subject to desiccation. t 



Lipman and Burgess have found that many soils manifest a vigor- 

 ous nitrogen-fixing power even after being air-dried and kept in 

 stoppered museum bottles for periods varying from five to twenty 

 years. In some cases the fixation was equally as high as in freshly- 

 collected samples. The organisms from such soils are more easily 

 attenuated than are other organisms which have not been so dried. 

 The tendency is for soils gradually to decline in nitrogen-fixing 

 power or drying. This may manifest itself as early as the second 

 week. 



x During the periods of drying, the organisms are inactive, as they 

 require moisture for growth and reproduction. For maximum 

 nitrogen-fixation a definite moisture content is needed. Warmbold 

 found the optimum moisture content to be 20 per cent. , When it was 

 below 10 per cent, there was no nitrogen fixed, and in some cases 

 there was a decided loss of nitrogen, Krainsky allowed soil with 



