108 



A(iKNOMV 



the ammonium hydroxide into nitrous acid and hydrogen by the 

 addition of an atom of oxygen (NH 4 OH + O = HNO 2 + H 4 ). 

 The nitrous acid combining with various minerals in the soil 

 form nitrites, and a third set of bacteria, of the genus Nitro- 

 bacter, now adds another atom of oxygen to this compound, 

 forming the nitrates used by plants. Although nitrites and 

 nitrates differ principally in the possession of one more atom 

 of oxygen by the latter, plants seem able to use only the 

 nitrates. Nitrobacter, so far as known, is the only bacterium 

 that can turn nitrites into nitrates. 



Nitrogen fixation. Certain plants known as legumes, of 

 which the bean, pea, clover, and alfalfa are examples, have the 

 power to fix atmospheric nitrogen, or, rather, they have set up 



a partnership with bac- 

 teria which are able to 

 do so. In this partner- 

 ship the bacteria (Pseu- 

 domonas radicicola), in 

 return for the carbohy- 

 drates which they need, 

 give to the legumes the 

 nitrogen which they are 

 able to take from the 



air. Such a partnership as this is called symbiosis, and the part- 

 ners are known as symbionts. Certain algae in the soil seem 

 also to have entered into symbiotic relations with bacteria for 

 the purpose of getting nitrogen. By carefully digging up 

 any legume and washing off the soil clinging to the roots the 

 nodules which the bacteria inhabit are easily seen. Another 

 bacterium, Azotobacter chroococcum, appears to be able to fix 

 atmospheric nitrogen by itself, oxidizing carbohydrates in the 

 soil in the process. The fact that lands allowed to lie without 

 cultivation, or fallow, for a time, increase in nitrogen content 

 is attributed to the presence of this and other bacteria. The 



FIG. 85. Nodules on the roots of a legume 



