SOIL 697 
plants contain pleomorphic organisms, B. rarlicicola, which were 
able to fix atmospheiic nitrogen. ]\Iaze^ aiul others have confirmed 
this observation. Somewhat later Winogradsky- isolated an anaerobic 
spore-forming bacillus, Clostridium pasteurianum, not depending upon 
plants for its sustenance, but free living, which accomplished the 
same transformation, and in 1901 Beijerinck^ isolated and described 
the very important group of Azobacteria, which are widely distributed 
in the soil and are able to fix atmospheric nitrogen. These organisms 
are most active when associated with other soil bacteria, but are fully 
able to fix nitrogen when grown in pure culture in artificial media free 
from nitrogenous compounds. 
The oxidation of ammonia salts to nitrites and then to nitrates is 
effected through the activities of nitrifying bacteria, first isolated and 
described by Warrington and Winogradsk>'. Two organisms are 
concerned, a coccus, Nitrosococcus, which transforms ammonium 
salts to nitrites, and a small bacillus, Xitrobacter, which oxidizes 
nitrites to nitrates. These organisms do not thrive in the presence of 
complex organic matter and appear to derive their nutritive require- 
ments chiefly from inorganic salts. The nitrates are taken up by 
chlorophyll-bearing plants and, with the energy of sunlight transform 
them, together with carbon dioxide, water, phosphates and various 
salts, into the complex vegetable proteins upon which the animal king- 
dom primarily subsists. 
It is obvious, therefore, that there is a well-defined nitrogen cycle— 
an intricate series of changes which proteins and their derivatives 
undergo, through which complex, lifeless nitrogenous compounds 
are reduced through bacterial activity to simple, stable mineralized 
inorganic combinations of their elements. These elements are restored, 
chiefly through the synthetic activity of plant life, to the animal king- 
dom. The nitrogen cycle is, in a sense, a measure of the metabolism 
of the living earth, in which the anabolic or synthetic processes occur 
in plants and indirectly in animals; the catabolic or analytic process 
is brought about chiefly by bacteria. 
In addition to the normal bacterial flora of the soil and adventitious 
saprophytic organisms, pathogenic bacteria are occasionally found; 
B. typhosus, dysentery and cholera organisms and other excrementi- 
tious bacteria are occasionally deposited on the ground with human 
excrement. These microorganisms do not, as a rule, survive prolonged 
exposure to air, sunlight and other unfavorable environmental vicis- 
situdes, however. Certain spore-forming bacteria— B. tetani, anthrax, 
symptomatic anthrax, Vibrion septique (malignant edema) and gas 
bacilli are very common in certain places. These bacteria, except 
anthrax, appear to multiply in the intestinal tracts of the herbivora. 
The natural or biological degradation and mineralization of dead 
1 Ann. Inst. Pasteur, 1897, 11, 44; 189.S, 12, 1, 128; 1899, 13, 145. 
2 Compt. read. Soc. de biol., 1893, 116, 1385; 1894, 118, 353. 
' Centralbl. f. Bakteriol., II Abt., 1901, 7, 562. 
