No. S. DEPARTMENT OF AGRICULTURE. 359 



monia and free nitrogen. Where the thickness was 60-70 mm. no 

 ammonia was formed, but only free nitrogen. 



Burri, Herfeldt and Stutzer,^*^ in 1S95, isolated from horse manure 

 and from straw two bacilli, respectively B. niti'ificans I and II, which 

 actively reduced nitrates. 



Schirokikh," in 1893, isolated from horse dung a bacillus which 

 liquified gelatin, and actively reduced nitrates. In broth contain- 

 ing 2.0 grams of potassium nitrate to the litre, the latter was com- 

 pletely reduced in 5.8 days, at a temperature of 30-35 degrees C. 



Again, Sewerin,^^ in 1897, isolated from horse manure 29 species of 

 bacteria, of which 20 possessed greater or less power of reducing 

 nitrates. 



In 1S96, Kichards and Eolfs*"^ conducted some experiments with 25 

 different solutions prepared to typify conditions of water polluted 

 with decaying organic matter (sewage), and at the same time con- 

 taining nitrates. 



They note, (1) the rapid disappearance of nitrates, usually less 

 than 10 per cent, of the original quantity remaining at the end of 

 3 days; (2) a corresponding increase of nitrites; (3) that when the so- 

 lutions contained no organic matter other than that usually present 

 in the water reduction took place very slowly and incompletely; (4) 

 that the nitrogen which disappeared from the nitrates was finally 

 given off in the free state; (5) that whenever nitrates were added to 

 decomposing organic matter under such conditions that the growth 

 of the bacteria required more oxygen than the solution afforded, the 

 latter took it from the nitrates setting free nitrites, which in time 

 were decomposed, setting free nitrogen. 



From the aforegoing citations it is seen that denitrifying bacteria 

 are abundantlv distributed in nature, and that thev are found in 

 water, soil, manure, sewage and upon the surface of plants, particu- 

 larly upon straw. It is only necessary to add a small portion of soil 

 to media containing nitrates to obtain active denitrification thus 

 showing the general Dresence of denitrifying bacteria in soils. 



The majority of soil bacteria studied separately by the author 

 possess this property to a greater or less degree. 



7. Conditions Affecting Denitrification, 



{a.) Presence or Absence of Air . — It has commonly been supposed that 

 the power of reducing nitrates belongs more exclusively to the an- 

 aerobic bacteria, or those which live without air. The question is 

 an open one as to how far denitrification is the result of a deficient 

 supply of atmospheric air. All of the species of soil bacteria so far 



