178 PLANT GROWTH 



nitrogen-fixing bacteria, removes more nitrogen from the 

 soil than is appHed as fertihzer. The legumes include the 

 clovers, vetches, beans, peas, lupins, etc., all of which de- 

 velop small ball-like nodules on their roots if the proper 

 bacteria are in the soil. A single strain of bacteria will inocu- 

 late several but not all species of clovers. Another strain is 

 necessary for lupins and a third for alfalfa. 



If the proper bacteria are not in the soil they must be 

 added to the seed or the soil. These symbiotic bacteria can 



AIR 79% NITROGEN ANIMALS MINERAL 



FERTILIZERS 



-4 



'' 



SOIL 



LEGUME LEGUME EXCREMENT PLANTS ^ NITRATES 



BACTERIA *"*" PLANTS 



/ 



FREE LIVING BACTERIA AMMONIUM -♦NITRITES 



SOIL BACTERIA — > DECOMPOSE -> COMPOUNDS 



ORGANIC MATTER "*-"-^ LEACHES 



FROM SOIL 



Fig. 25. The nitrogen cycle. Nitrogen from the air may be traced by the arrows 

 as it is acted upon by various bacteria until it can be used by plants. The plants may 

 be eaten by animals or they may be decomposed by bacteria. 



convert free nitrogen from the air into a molecule which can 

 be used by the legume, and when the legume is decomposed 

 by other bacteria the molecular nitrogen remains in the soil. 

 This and the later described bacterial action are shown in the 

 nitrogen cycle (Fig. 25) which should be consulted as a guide 

 to the whole problem. 



A legume crop removed for hay will not add so much 

 nitrogen to the soil as the corn crop uses. Thus crops remove 

 more nitrogen from the soil than is added by man, and leach- 

 ing may lose a like amount. A small amount is washed from 

 the atmosphere as ammonia by rains. 



Two sources may explain the problem of the additional 

 supply: first, the work of the free-living nitrogen-fixing bac- 



