CHAPTER THIRTY-EIGHT 



SOIL BACTERIA AND THE NITROGEN CYCLE 



Next to carbon, hydrogen, and oxygen the most important 

 element used by plants is nitrogen. Agricultural crops on 

 mineral soils are very frequently limited by an insufficiency of 

 this element. As we shall see, the occurrence of nitrates in soils 

 is due almost entirely to the action of bacteria and fungi. Owing 

 to differences in their modes of life, several groups of nitrogen 

 bacteria are distinguished, all of which play an important role 

 in the nitrogen cycle in nature. 



Nitrifying bacteria. In order to manufacture proteins, seed 

 plants must have a supply of nitrogen, usually in the form of 

 nitrates. There may be other nitrogen compounds in the soil, 

 but they are for the most part unavailable until certain nitrifying 

 bacteria change them to nitrates. Ammonia is one of the nitro- 

 gen compounds produced in the process of humus formation. 

 If the soil is moist, the temperatures high, and the drainage 

 sufficient to provide an adequate air supply, ammonia will be 

 acted upon by certain bacteria and changed to nitrites, which in 

 turn are changed by other bacteria into nitrates. These are 

 oxidizing processes, and the energy liberated is used by the nitri- 

 fying bacteria in the various chemical syntheses necessary to 

 transform CO2 and H2O and the soil salts into food and into 

 tissue substances. These plants are as truly autophytic as the 

 complex green plants. They require nothing but inorganic 

 substances to maintain themselves. Wherever ammonia occurs, 

 the nitrifying bacteria soon make it available for green plants. 



Saprophytic nitrogen-fixing bacteria. Still other bacteria bring 

 about a process known as nitrogen fixation, by which nitrogen is 

 actually taken from the air and built into compounds which 

 are added to the soil. The nitrogen-fixing bacteria are, with a 

 few exceptions, the only plants that can take nitrogen from the 

 air and combine it to form nitrogen compounds. They flourish 



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