CHAPTER XLIII. 



BACTERIA OF THE NITROGEN CYCLE FERMENTATION OF UREA 

 NITRIFYING AND DENTRIFYING ORGANISMS- 

 FREE NITROGEN FIXATION. 



THE importance of the nitrogen cycle in its relation to the main- 

 tenance of vegetable and animal life has already been briefly referred 

 to in Chapter II. It was there stated that bacteria and other low 

 vegetable organisms perform a most important function of this cycle. 

 The subject will now be taken up in greater detail in the discussion 

 of the particular bacteria concerned in these phenomena. 



Plants, which are ultimately necessary for the preservation of 

 animal life, obtain their nitrogen from the soil, chiefly in the form of 

 nitrates (i. e., salts of nitric acid), and to only a small extent in the 

 form of ammonia salts. As the amount of nitrates in the soil is limited, 

 it would soon become sterile if the nitrates were not replaced. The 

 nitrates taken up by the higher plants are changed in their metabolism 

 into vegetable proteids, such as the gluten of grasses and the legumen 

 of leguminosse (clover, peas, beans, lentils, etc.), and these' in turn 

 when taken up as vegetable food are changed into other proteids in 

 the animal body. Man and the lower animals excrete the nitrogen 

 waste mainly in the form of urea. Lafar states that mankind excretes 

 daily about 37,500 tons of urea, containing seventeen million kilo- 

 grams of combined nitrogen, while animals certainly excrete a much 

 larger daily amount of this nitrogenous waste product. Urea cannot 

 be directly utilized by our cultivated plants and the question, therefore, 

 arises, How does this animal nitrogen-containing waste product 

 become available again as a source of nitrogen for the building up 

 of vegetable proteids? It is through the decomposition of the urea 

 by the action of bacteria and the change of this decomposition product 

 by other bacteria, making it, finally, again utilizable in the nutrition 

 and metabolism of plants. By decomposition is meant the breaking 

 down of bodies of a more or less complex chemical composition into 

 simpler compounds ; in other words, the changing of a complex mole- 

 cule into two or more simpler molecules. To illustrate, the following 

 examples may be cited : If some yellow oxide of mercury is heated in 

 a test-tube it is decomposed into metallic mercury and oxygen. If a 

 proteid is exposed to the action of pure boiling sulphuric acid in a 

 so-called Kjeldahl flask this very complex body is decomposed into 

 carbon dioxide, water, ammonia, and other simple molecules. What is 

 accomplished with the proteid by artificial chemical manipulation is 



