274 THE FERTILITY OF THE SOIL 



formed by the bacteria. It is also one which yields energy, 

 which the organisms use to change CO2 or some carbonate 

 (e.g., CaCOa, calcium carbonate) into such compounds as 

 are useful to them in carrying on their life activities. 



The Nitrate Bacteria cannot use ammonia or its com- 

 pounds, but are restricted to the nitrites formed by the 

 organisms discussed in the last paragraph. Presumably 

 by means of an enzym they oxidize nitrous acid and its 

 salts to nitric acid and its salts somewhat as follows: 

 2HNO2 (nitrous acid) + O2 (oxygen) = 2HNO3 (nitric 

 acid). 



Nitrifying Organisms and Higher Plants. — The 

 contrast between these two sorts* of nitrifying bacteria 

 on the one hand and the higher plants and other bacteria 

 on the other is a striking and important one. Higher 

 plants depend on complex carbon compounds for the 

 energy to carry on their life processes (i.e., metabolism). 

 They use some of this energy to transform simple nitrogen 

 compounds like nitrates into the vastly complex ones 

 which constitute an important part of their protoplasm. 

 These bacteria do just the opposite. They can use only 

 simple compounds of carbon like COo. This they build 

 into such complex forms as they require by the energy 

 derived from oxidizing nitrogen compounds. Further- 

 more, they leave the nitrogen in just those compounds 

 which are required by the higher plants. 



Effects of Cultivation. — Both sorts of nitrifying 

 bacteria work best in soils with an alkaline reaction and 

 are injured by wet acid soils. For this reason they usually 

 thrive under cultivation because this tends to secure 

 proper aeration and suitable distribution of moisture, and 

 I^revents the formation and accumulation of injurious 

 acids. 



The Nitrogen-fixing Bacteria are so called because 

 they take free atmospheric nitrogen and unite it with 

 hydrogen and oxygen from water to form complex com- 

 pounds. This, reaction is one in which energy is stored 



