Oxygen 

 in 

 air 



Carbon 

 dioxide 



in 



air 



K't 



>/ 



»y 



Bodie 



Carbon 



and 

 oxygen 



m 



<:>'. 



,ee5 



^y 



soil 



^! 



.^1^ 



^5 



,^^ 



Bodies 



Excretion 



^Q'j 



:'o.<f 



^j^ 



Herbivores 



.;s.j 



A- 



Omnivores 



*^., 



Food 



Carnivores 



THE CARBON-OXYGEN CYCLE 



The material of green plants consists in part of carbon derived from the carbon diox- 

 ide of the air. This carbon is passed on to animals as food, or returned to the air 

 by respiration or by burning. Animals either pass on the carbon to other animals 

 which eat them, or return it to the air by respiration. Some of the carbon is locked 

 temporarily in the soil as excretions or as dead bodies. Through decay, the action 

 of bacteria and fungi, this carbon is returned to the air as carbon dioxide 



water, through the soil and other organisms. And while the atmosphere 

 is nearly four-fifths uncombined, or "free", nitrogen, green plants cannot 

 utilize it. 



As a matter of public economy, people have found it worth while to save 

 the manure of barnyards and even the sewage of cities for the nitrogen com- 

 pounds that these contain. But in spite of all our saving, vast quantities of 

 nitrogen are washed out to sea or thrown into the air beyond the reach of 

 our common plants. 



It has been possible to use nitrates, which are found as mineral deposits 

 in certain places, especially in Germany and Chile. But the quantity of these 

 nitrates is limited, and they are relatively expensive. On certain islands off 

 the coast of South America there are extensive deposits of guano, or bird 

 refuse, left there by countless birds that have built their nests upon these 



150 



