BACTERIA OF THE SOIL 261 



tions at least, of fixing atmospheric nitrogen and utilizing 

 it for their own metabolism. Some species of these are ana- 

 erobic, most are aerobic. As noted above with EhizoMum, 

 it is customary to rate the rapidity and amount of nitrogen 

 fixation by comparing the number of milligrams of nitrogen 

 fixed with the number of grams of sugar utilized as energy, 

 for these organisms are just as dependent as are the sym- 

 biotic type upon some easily oxidizable substance, such as 

 carbohydrates, from which they may secure energy. The 

 group of organisms which has been most studied and is 

 probably, on the whole, most important in such nitrogen 

 fixation, is the one whose i^embers belong to the genus 

 Azotobacter. 



Azotobacter. — The genus Azotobacter includes those soil 

 organisms which are aerobic, secure their growth energy 

 almost entirely by the oxidation of carbohydrate material 

 and which are capable of fixing atmospheric nitrogen. The 

 cells usually are somewhat larger than those of other soil 

 bacteria. Under favorable conditions in the laboratory in 

 suitable media, they may appear almost yeastlike. They 

 are usually spherical, ellipsoid, or in some cases elongate. 

 Some species are motile, usually by means of polar flagella. 

 Others are nonmotile. Some species possess the power of 

 producing a diffusible pigment, greenish or brown in color. 

 Species belonging to this genus are widely distributed in 

 nature and about ten have been described in the literature. 



The species first described, and one of those most com- 

 monly met with is Azotobacter cJiroococcum. This and 

 another species known as Azotobacter Beyerincki are prob- 

 ably the most widely distributed in soils. They grow 

 readily upon laboratory media containing phosphate and 

 certain other inorganic salts plus some easily available 

 source of carbon, particularly sugars and higher alcohols. 

 They do not develop in media or in soils having an acid 

 reaction. One species of Azotobacter, A. Yinelandii, de- 



