186 PRINCIPLES OF SOIL MICROBIOLOGY 



of an acid, nitrate reduction continues further, until all the nitrate has 

 disappeared. 38 The organisms are very sensitive to free acids. The 

 optimum reaction for the reduction of nitrates is pH 7.0 to 8.2; the limit- 

 ing reactions are pH 5.5 and pH 9.8. The optimum reaction for the 

 reduction of nitrites is pH 5.5 to 7.0. 



The reduction of nitrates to atmospheric nitrogen may be a result of 

 associative action of two bacteria, one (Bad. coli) reducing the nitrate 

 to nitrite and the other (Bad. denitrificans I) reducing the nitrite to 

 atmospheric nitrogen. 37 In case of associative growth, the aerobic 

 form removes the free oxygen, thus enabling the other organism to be- 

 come the denitrifier. Some organisms reduce only nitrates to nitrogen. 

 The four species found by Maaszen capable of reducing nitrate to gaseous- 

 nitrogen were Bad. fluorescens liquefaciens, Bad. fluorescens from blood, 

 Bad. pyocyaneum and Bad. praepollens. These results were confirmed 

 by other investigators, 39-41 who found Bad. pyocyaneum, Bad. hartlebii 

 and fluorescent bacteria among the most active denitrifying organisms. 



Among the forms capable of reducing nitrates completely to gaseous- 

 nitrogen, we may also include various organisms isolated from horse 

 manure, 42 from cattle excreta (Bad. denitrificans agilis) i3 and from the 

 soil. 44-46 Van Iterson 47 demonstrated the presence in the soil of various 

 bacteria, namely Bad. stutzeri, Bad. denitrofluorescens and Bad. vul- 



38 Zacharowa, T. M. Process of denitrification as dependent upon the reaction 

 of the medium. Trans. Institute of Fertilizers, No. 15, 1923, Moskau. 



39 Sewerin, S. A. Zur Frage fiber die Zersetzung von salpetersauren Salzen 

 durch Bakterien. Centrbl. Bakt. II, 3: 504-517, 554-563. 1897; 22: 348-370. 

 1909; 25: 479-492. 1909. 



40 Christensen, H. R. Zwei neue fluoreszierende Denitrifikationsbakterien. 

 Centrbl. Bakt. II, 11: 190-194. 1904. 



41 Fred, E. B. Eine physiologische Studie iiber die nitratereduzierenden 

 Bakterien. Centrbl. Bakt. II, 32: 421-449. 1911. 



42 Schirokikh, J. Uber einen neuen Salpeter zerstorenden Bacillus. Centrbl. 

 Bakt. 11,2: 204-207. 1896. 



43 Ampola, G., and Garino, E. Ueber die Denitrifikation. Centrbl. Bakt. II, 

 2: 670-676. 1896; 3: 309-310. 1897. 



44 Jensen, 1897-8 (p. 180). 



45 Hoflich, C. Vergleichende Untersuchungen iiber die Denitrifikationsbakter- 

 ien des Mistes, des Strohes und der Erde. Centrbl. Bakt. II, 8: 245-248, 273-278, 

 305-308, 336-339, 361-367, 398-406. 1902. 



46 Cingolani, M. Recherche intorno al processo della denitrificazione. Staz. 

 Sper. Agr. ital. 41: 521-538. 1908; Ann. Staz. Chim. Agr. Spes. Roma (2), 2: 

 274. 1908. (Centrbl. Bakt. II, 23: 238. 1909.) 



47 van Iterson, C. Anhaufungsversuche mit denitrifizierenden Bakterien. 

 Centrbl. Bakt. II, 12: 106-116. 1904. 



