74 Biology of Micro-organisms 



The Formation of Nitrates. A process of fundamen- 

 tal importance is carried on by certain lowly bacteria of 

 the soil. Since plants are unable to assimilate the free 

 nitrogen of the air, but must obtain this element from 

 the soil in the form of some soluble compound, and since 

 there is a relatively limited amount of combined nitrogen 

 in the world, it becomes of the last importance that the 

 supplies which are continually withdrawn from the soil 

 should be replaced by the nitrogen liberated in the decay 

 of organic material. This nitrogen, after a series of putre- 

 factive changes have occurred, appears as ammonia. The 

 odor of this gas is often plainly perceptible about manure 

 heaps. In this form nitrogen is poorly adapted for use by 

 plants, and moreover may be easily dissipated. An ex- 

 tensive further process of oxidation is carried on by the 

 nitrifying bacteria, whereby nitrates are ultimately formed. 

 These are eminently adapted for use by plants, and so the 

 soil is rendered continuously capable of supporting vege- 

 tation. 



Nitrosomonas and Nitrosococcus convert ammonia into 

 nitrous acid, and Nitrobacter oxidizes the latter to form 

 nitric acid. 



These genera are well nigh universal in the soil. They do 

 not grow on the ordinary culture media, but require special 

 solutions, free from the diffusive albumins, free, indeed, 

 from organic compounds of any sort. Their supplies of 

 carbon are obtained by the dissociation of carbon dioxid. 

 It is highly noteworthy that they are thus able to flourish 

 without food more complex than ammonia, a fact which is 

 without parallel among organisms devoid of chlorophyl. 



Reduction of Nitrates. A considerable number of bac- 

 teria are able to reduce nitrogen compounds in the soil 

 or in culture media, prepared for them, into ammonia. 

 To the horticulturist this matter is of much interest. 

 Winogradsky * has described specific nitrifying bacilli which 

 he found in soil, and asserts that the presence of ordinary 

 bacteria in the soil causes no formation of nitrites so long 

 as the special bacilli are withheld. 



Reduction of nitrates can be determined experimentally by 

 the use of a nitrate broth, made by dissolving in 1000 c.c. of 

 water i gram of peptone and 0.2 gram of potassium nitrate. 

 The ingredients are dissolved, filtered, then filled into tubes, 



* "Ann. de 1'Inst. Pasteur," 1891; "La Semaine medicale," 1892. 



