SOIL 617 



compounds suitable for plant food. It is essential to relate in some 

 detail the manner in which these transformations are accomplished. 



The amount of nitrogen available at the present time for synthesis 

 by plants exists chiefly in an organized state, and as nitrates in the 

 soil. Nitrates are very soluble and it is obvious that large amounts 

 of available nitrogen are yearly carried in solution to the ocean where 

 they are practically lost. Brandt 1 estimates this loss to be about 

 40,000,000 kilograms annually. It is obvious that this loss must be 

 compensated for. 



It is a matter of common observation that soil left uncultivated 

 gains in fertility from year to year and in 1875 Barthelot, and Nobbe 

 and Hiltner 2 made the important discovery that nitrogen from the 

 air is fixed in the soil. It was found that soil heated to 100 C. lost 

 its power of fixation of nitrogen, suggesting that microorganisms 

 played a part in the process. In 1888 Beijerinck 3 made the very 

 important discovery that nodules 4 upon the roots of leguminous 

 plants contain pleimorphic organisms, Bacillus radicicola, which 

 were able to fix atmospheric nitrogen. Maze 5 and others have con- 

 firmed this observation. Somewhat later Winogradsky 6 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 7 iso- 

 lated 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 Winogradsky. Two organisms are 

 concerned, a coccus, Nitrosococcus, which transforms ammonium 

 salts to nitrites, and a small bacillus, Nitrobacter, which oxidizes 

 nitrites to nitrates. These organisms do not thrive in the presence 

 of complex organic matter and appear to derive their nutritive require- 



1 Report Kommission zur Untersuch. d. deutsche Meere, 1899-1901. 



2 Landwirthsch. Versuchsstat, xlv. 



3 Bot. Zeitung, 1888, 725. 



4 These nodules were first described by Hellriegel (Tageblatt Naturforsch. Vers., 

 Berl., 1886, 290) and Willforth (ibid., 1887, 362). 



s Ann. Inst. Past., 1897, xi; 1898, xii. 



6 Compt. rend. Soc. biol., 1893, cxvi, 1385; 1894, cxviii, 353. 



7 Centralbl. f. Bakt., 1901, vii, 562, II Abt. 



