CHAPTER VIII 

 THE CYCLE OF CARBON 



The cycle of carbon may be best presented in the decom- 

 position of the carbohydrates. The carbon in fats and in 

 proteins is ultimately changed into the same form as in 

 carbohydrates, e. y. carbon-dioxide. Many of the decom- 

 positions of carbohydrates are of industrial importance and 

 will he discussed later. Others can most conveniently he 

 considered in connection with the soil processes. 



During the growing season, the green plants draw heavily 

 on the carbon-dioxide in the air. It is, of course, clear 

 that, if agencies are not operating to return the carbon to 

 the air as carbon-dioxide, the yrowih of green plants will 

 ultimately be limited. The air contains from 0.03 to 0.04 

 per cent, of carbon-dioxide. 



Every living form produces carbon-dioxide through its 

 respiratory processes. The green plant during daylight 

 hours is both producing and consuming this gas; the former 

 is accomplished in respiration, the latter by photosynthesis. 

 The two processes tend to balance each other. In the dark 

 photosynthesis is stopped, while carbon-dioxide production 

 goes on. All animals and fungus plants produce carbon- 

 dioxide. The green plant robs the air of its carbon-dioxide ; 

 the rest of living things replace it, and thus life is able to 

 continue its round. The decomposition of organic matter 

 taking place in the soil abstracts oxygen from the soil 

 air, and increases its content in carbon-dioxide, until as 

 much as from 2 to 9 per cent, of this gas may be found in 

 the soil. Poor ventilation of the soil tends to maintain the 



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