262 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



scribed by Lipman in New Jersey, has been found able, in 

 the laboratory, to fix from fifteen to twenty milligrams of 

 nitrogen per gram of sugar or alcohol (such as mannitol) 

 used. The amount of nitrogen fixed is also influenced by 

 other constituents of the medium. It has been shown by 

 certain authors that the addition of soil extract, particularly 

 of humates, exerts a decided stimulating action. It has been 

 found that under certain conditions species of Azotobacter 

 growing together with algas are capable of fixing consider- 

 able quantities of nitrogen, apparently there existing sym- 

 biotic relationship between the two types of organisms. 



Efforts have been made to utilize cultures of Azotobacter. 

 Most fertile soils contain bacteria of this type in abundance. 

 The addition of such organisms, therefore, is superfluous in 

 such soils and useless in soils that are not adapted to their 

 growth. 



Organisms belonging to this group are not of such im- 

 mediate value to the farmer as are those growing in sym- 

 biosis with leguminous plants. Nevertheless, in the long 

 run they are probably responsible for the accumulation and 

 storing up of large quantities of nitrogen in the soil. It 

 should be noted, however, that there is no evidence that 

 these bacteria take up free nitrogen and turn it over to 

 plants growing in the soil. It is probable that they utilize 

 the nitrogen in their own metabolism and it is only after the 

 death and decay of the Azotobacter cells that the nitrogen 

 becomes available to higher plants. Inasmuch as these 

 bacteria can utilize sugars, and salts of various organic 

 acids as sources of energy in the laboratory, it is probable 

 that they secure their energy from the same sources in the 

 soil. Their development, therefore, is dependent upon an 

 abundant supply of such materials. 



Anaerobic Nitrogen-fixing Bacteria. Several species of 

 anaerobic bacteria have been described as capable of fixing 

 atmospheric nitrogen. The most important of these is 



