AUTOTROPHIC BACTERIA 77 



tion are inseparable functions; organic matter (1 per cent peptone) may 

 paralyze the organism, but does not kill it and does not change it, since, 

 when transferred upon proper media, it resumes its activities. 



The respiration of the organisms, the chemistry of the processes of 

 nitrite and nitrate formation, and their importance in soil fertility 

 are discussed in detail elsewhere (p. 525). 



Occurrence of nitrifying bacteria in the soil. All soils, not very acid in 

 reaction, contain bacteria capable of oxidizing ammonium salts to 

 nitrites and the latter to nitrates. The limiting acidity for the develop- 

 ment of these bacteria in the soil is pH 4.0 to 4.4, while the optimum 

 reaction is at pH 6.8 to 7.3. 36 When a soil more acid in reaction than 

 the minimum for their development is treated with lime, the or- 

 ganisms will gradually appear in the soil; however, inoculation with 

 a good fertile soil is often practiced, so as to introduce the or- 

 ganisms immediately. The numbers of the nitrifying bacteria per 

 gram of soil vary from a few to 10,000. 37 The method commonly used 

 for this determination consists in diluting the soil with sterile water, 

 then adding 1 cc. portions of the various dilutions to the proper media. 

 Positive growth indicates a minimum number of organisms. It is 

 possible, however, that many cells have to be added to a liquid medium, 

 before growth can take place, since conditions are not made as favorable 

 for their development in artificial liquid media as in normal soil. In 

 humid soils, the bacteria are present in the upper few inches and 

 rapidly disappear in the subsoil. However, in arid soils, they occur to 

 a depth of many feet. 38 



In addition to the typical nitrite and nitrate bacteria described by 

 Winogradsky and isolated by other investigators, various reports have 

 been made concerning the isolation of nitrite and nitrate forming 

 organisms, ranging from typical autotrophic forms, like the nitro- 

 microbium of Stutzer and Hartleb, 39 to forms possessing properties 

 altogether uncharacteristic of autotrophic bacteria, such as cellulose 

 decomposition, gelatin liquefaction, nitrate reduction, etc.; 40 however, 

 the last investigations still remain to be verified. 



36 Gaarder, T., and Hagem, O. Nitrifikation in sauren Losungen. Bergeng 

 Museum Aarbook. 1922-3, No. 1. 



"Duggeli, 1923 (p. 39). 



38 Lipman, 1912 (p. 36). 



39 Stutzer, A., and Hartleb, R. Untersuchungen fiber die bei der Bildung von 

 Salpeter beobachteten Mikroorganismen. Mitt, landw. Inst. Breslau, 1: 75-99, 

 197-232. 1901. 



"Sack, J. Nitratbildende Baktcrien. Centrbl. Bakt., II, 62: 15-24. 1924; 

 64: 32-37, 37-39. 1925. 



