60 



NATURAL HISTORY 



fjring' 

 JoactcHee 



Bacteria also act upon ammonia formed from plant and animal 

 wastes, one kind (Nitrosomonas) producing nitrites, or nitrate salts, 

 and others (Nitrobader) converting the nitrites into the more stable 

 nitrates. Thus all of the compounds of nitrogen are used over and 

 over, first by plants, then as food by animals, eventually returning to 

 the soil, or in part being released as free nitrogen. This process is 



called the nitrogen cycle. 

 Although free nitrogen is 

 fixed for use by means of 

 electrical discharges dur- 

 ing thunderstorms, by 

 man-made machines, by 

 ultraviolet light (which 

 is estimated to return 

 100,000,000 tons a year 

 to the earth's surface), 

 and from other sources, 

 yet these means give an 

 almost negligible amount 

 of usable nitrogen to the 

 soil, compared with what 

 is used in crop produc- 

 tion, especially since so much nitrogen is lost from the soil in various 

 ways. The nitrogen-fixing bacteria supply the deficiency, thus form- 

 ing one of the most important inter-relationships between plants and 

 animals because of their direct relationship to the production of the 

 food of the world. 



Rotation of Crops 



Plants that are hosts for the nitrogen-fixing bacteria are raised 

 early in the season, then plowed under and a second crop of a differ- 

 ent kind is planted. The latter grows quickly and luxuriantly be- 

 cause of the nitrates left in the soil by the bacteria which lived with 

 the first crop. For this reason, clover is often grown on land used 

 later for corn, or cowpeas will be followed by a crop of potatoes. 

 On well-managed farms, different crops are planted in succession 

 in a given field in different years so that one crop may replace some 

 of the elements taken from the soil by the previous crop. This is 

 known as rotation of crops. ^ 



' Crop rotation is not only a process to conserve the fertility of the soil, but also a sanitary meas- 

 ure to prevent infection of the soil. 



The nitrogen cycle. What additions could 

 made to this diagram? 



be 



