METABOLISM 75 



of the carbon, hydrogen, - and nitrogen in living 

 tilings, and in consequence there is little danger of 

 the supply ever becoming exhausted. The purpose 

 of phosphate fertilization is to supply the demands 

 of the nitrogen-fixing bacteria (Azotobacter) , rather 

 than the green plants themselves, and thus to aid 

 the process of nitrogen fixation (see below). 



Denitrification. - - The circle, nevertheless, is not 

 so ideal as it might seem, for there exists another 

 group of bacteria in the soil whose special activity 

 it is to reduce nitrates again to gaseous nitrogen, 

 which escapes to the atmosphere and is added to 

 the inert quantity that plants are unable to "fix" 

 or utilize. This means a constant loss of available 

 nitrogen, a constant deficit, so to speak, in the annual 

 balance sheet, and when the facts first became 

 known, considerable doubt was expressed as to the 

 future habitability of the earth when the available 

 nitrogen should have been reduced too far. For- 

 tunately for our peace of mind, there have been dis- 

 covered yet other kinds of bacteria and molds that 

 are capable of fixing, that is, combining with the 

 nitrogen of the soil. These seem to be everywhere 

 present in soils and make up for the loss due to the 

 denitrifying bacteria. 



It has been known for many centuries that it im- 

 proves the land to grow crops of peas, beans, or their 

 relatives, and plow them under as " green manure." 

 The ancient Romans and the Chinese and Japanese 

 carried out such practices without knowing any 



