June 2,1923 
Nitrogen Fixation by Azotobacter 
767 
SUMMARY 
The data presented in this paper point very definitely to a limiting 
hydrogen-ion concentration of Ph 5.9 to 6.0 for the various cultures of 
Azotobacter employed when grown under the conditions of these experi¬ 
ments. Vigorous growth and nitrogen fixation took place at Ph 6.1 to 
6.5, the optimum Ph for nitrogen fixation apparently being somewhat 
higher than the optimum for growth. Very slight, if any, changes in 
the reaction of the media are produced by the growth of the various 
strains of Azotobacter studied, indicating the production of inappreciable 
quantities of acid or basic metabolic by-products. 
The data for pure cultures here presented agree very closely with our 
findings in soil and tend to substantiate former conclusions that this 
group of organisms will not exist and function in soils, the hydrogen-ion 
concentration of which is greater than Ph 5.9 to 6.0. 
LITERATURE CITED 
(1) BeijKrinck, M. W., and Van Dkldbjn, A. 
1902. UEBKR DIB ASSIMILATION DBS FRBiBN STICKSTOFBS DURCH BAKTBRIBN. 
In Centbl. Bakt. [etc.], Abt. 2, Bd. 9, p. 3-43. Bibliographical foot¬ 
notes. 
(2) Bonazzi, Allgusto. 
1921. STUDIES ON AZOTOBACTER CHROOCOCCUM BEIJ. In JoUT. Bact., V. 6, p. 
331-369, 6 fig. References, p. 368-369. 
(3) Fred, E. B. 
1918. influence OF REACTION ON NITROGEN-ASSIMILATING BACTERIA. In JoUT. 
Agr. Research, v. 14, p. 317-336, i fig. Literature cited, p. 335-336. 
(4) Gainey, P. L. 
1918. SOIL REACTION AND THE GROWTH OF AZOTOBACTER. (PRELIMINARY 
PAPER.) In Jour. Agr. Research, v. 14, p. 265-271. 
(5) — 
1922. A STUDY OF SOME FACTORS CONTROLLING THE PRESENCE OF AZOTOBACTER 
IN SOILS, (abstract.) In Abs. Bact., v. 6, p. 14. 
(6) Lipman, J. G 
1905. SOIL BACTERIOLOGICAL STUDIES. FURTHER CONTRIBUTIONS TO THE 
PHYSIOLOGY AND MORPHOLOGY OF MEMBERS OF THE AZOTOBACTER 
GROUP. In 25th Ann. Rpt. N. J. Agr. Exp. Sta. 1904, p. 237-289, illus. 
(7) Stoklasa, Julius. 
1908. beitrag zur kenntnis der chemischen vorgange bei der assimila¬ 
tion DES ELEMENTAREN STICKSTOFFS DURCH AZOTOBACTER UND RADIO- 
BACTER. In Centbl. Bakt. [etc.], Abt. 2, Bd. 21, p. 484-509, 620-632, 
3pl- 
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