PROBLEM I . Why Plants and Ani?nals Continue to Exist 



animai 



in ammonia 

 compounds 



-N 



free in air 



Fig. 328 Find "N in nitrates" on the left and Fig. 329 Roots and base of peanut plant. Ni- 



trace an atom of nitrogen through one coin- trogen-fixing bacteria live in the nodules. How 



plete cycle. Which orgajiisms are involved in are legumes such as the pea?iut related to the 



this cycle? How is free N produced? jiitroge?i cycle? (Brooklyn botanic garden) 



the soil and kept in plant proteins, in ani- 

 mal proteins, and in the nitrogenous 

 wastes of animals. In all these forms ni- 

 trogen is useless to plants. 



The nitrogen, however, is not locked 

 up forever. The ammonium compounds 

 and nitrates needed by green plants are 

 restored to the soil by several groups of 

 bacteria and larger fungi. One group is 

 the decay organisms which form am- 

 monia from the proteins and urea. The 

 sharp odor from a decaying pile of com- 

 post or of manure is that of ammonia. 

 Some of the ammonium compounds are 

 used directly by green plants but large 

 quantities of them are built up by an- 

 other group of bacteria into nitrates. 

 The nitrify in g (ny'tri-fy-ing) bacteria 

 do this. A careful study of Figure 328 

 will help you understand how nitrogen 

 is kept in circulation. 



Complications in the nitrogen cycle. 



Side by side with the nitrifying bacteria 

 which build up nitrates in the soil are 

 other species of bacteria known as deni- 

 trify mg bacteria. Some of the denitrify- 

 ing bacteria again break down some of 

 the nitrates into ammonium compounds 

 and others break down nitrates com- 

 pletely and form nitrogen gas, which 

 then becomes a part of the air. The ele- 

 ment nitrosren cannot be used by ^reen 

 plants in food-making, so nitrogen thus 

 released is temporarily lost to green 

 plants and thus to animals. This loss of 

 nitrogen would be serious were it not 

 for still another group of bacteria. These 

 are the nitroge?i- fixing bacteria of the 

 soil. These bacteria use the gaseous nitro- 

 gen of the air and produce plant proteins, 

 which may be changed to nitrates by the 

 decay and nitrifying bacteria. Some of 



