THE INTER-RELATIONS OF PLANTS AND MAN 115 



and no animals can make use of this. The nitrogen must be in the 

 ground in the form of nitrates. Green plants remove tons of the 

 available nitrates from the soil each growing season, so that 

 eventually — as in the case of extraction of carbon dioxide from 

 the atmosphere — the supply becomes depleted. This is evident in 

 run-down farms where no more crops can be grown because the 

 nitrates have been exhausted and must be replaced by fertilizers 



NO, 



t ■ 



NITROGEN-FIXING 

 BACTERIA 



GREEN PLANTS 



N 



NITRATE BACTERIA 



NO. 



NITRITE BACTERIA 



NH, 



ANIMALS 



DECAY BACTERIA 



Fig. 77. — ^Bacteria are also important in the cycle of nitrogen. 



before crops can again be grown in the same soil. Nature con- 

 tinually re-fertilizes the soil from the remains of dead plants and 

 animals, again using the colorless plants in the cycle. Decay 

 bacteria which can "feed upon" proteins (the organic substances 

 which contain the nitrogen) decompose the plant or animal 

 remains with the evolution of ammonia as a waste product. The 

 ammonia is oxidized by two sets of bacteria which can utilize this 

 compound in their metabolic processes — the nitrite and nitrate 

 bacteria — and thus produce nitrates. The nitrates thereupon are 



