128 APPLIED BIOLOGY 



117. The Cycle of Nitrogen. Parallel with the cycle of 

 carbon from air to plants, from plants to animals, and from 

 animals back to the air, there is a corresponding cycle of the 

 element nitrogen. There is an abundance of this in the air, 

 but only certain species of bacteria can use it in its free form. 

 It has been pointed out that ordinary plants get nitrogen 

 in compounds absorbed from the soil and use this in making 

 proteins. Also, only plants can make the proteins required 

 by animals as food. Proteins in animals and plants, then, 

 depend upon the nitrogen in simple compounds such as are 

 found in fertile soil. The nitrogen in the bodies of plants 

 and animals gets back to the soil either by the decay of dead 

 animals and plants, or as excretions produced by animals ; 

 while animals obtain protein foods from plants directly or 

 indirectly through other animals. Animal excretions fur- 

 nish nitrogen to the soil, from the soil the nitrogen goes to 

 plants, from plants it goes to animals as protein food, and 

 then animals reduce the food to nitrogenous excretions, 

 which in the soil may again be taken up by plant roots. 



Such is the cycle of nitrogen, which, like that of carbon, 

 must be continuously undergoing repetition. If dead ani- 

 mals and plants did not decay and living animals not change 

 protein foods to nitrogeneous excretions, all the once avail- 

 able nitrogen would soon be bound in the bodies of dead ani- 

 mals and plants and life would cease. A marked tendency 

 toward such a result is evident in certain agricultural regions 

 where farmers have wasted manures (animal excretions) 

 until the soil has lost its primitive fertility; that is, has suf- 

 fered exhaustion of the nitrogen compounds. 



"118. Interdependence of Animals and Plants. We have 

 noted that animals depend upon food from plants for both 

 carbon and nitrogen, and that animals return these elements 

 to plants. Thus the cycle of organic matter is a double 

 one, including a cycle of carbon and one of nitrogen. 



The explanation of a " balanced " aquarium, one in which 



