OXIDATION PROCESSES IN THE SOIL 529 



bacteria the optimum is at 7.1 with limiting reactions at 6.5 and 7.8. 

 The kind of buffer used is of importance. 37 According to Gerretsen, 38 

 the limiting acid reaction is at pH 3.9 to 4.5, depending on the origin 

 of the nitrifying bacteria; those isolated from acid soils are better 

 adapted to acid conditions. The limiting alkali reaction was found to 

 be at pH 8.9 to 9.0. Meek and Lipman 39 demonstrated that the 

 limiting reactions for the nitrite and nitrate-forming bacteria may- 

 depend on the reaction of the soil from which they were isolated. By 

 gradual adaptation, the organisms can be made to grow at reactions 

 beyond their acid and alkaline range so that nitrate formation was even 

 obtained at pH 13.0. 



When ammonium sulfate is used as a source of nitrogen for nitrate 

 formation and the reaction of the soil is acid to begin with, there will be 

 an increase in acidity in the absence of sufficient buffer or base, as a 

 result of formation of nitric acid from the oxidation of the ammonia and 

 the accumulation of the residual sulfuric acid. Nitrate accumulation 

 will proceed until the reaction of the soil has reached a pH of about 

 4.0. The amount of nitrate formed under these conditions depends 

 upon the initial reaction of the soil and its buffer and base content; 

 the higher the buffer and base content of the soil, the larger will be 

 the amount of nitrate formed for a certain change of reaction. 40 The 

 continuous use of ammonium sulfate as a fertilizer without the addition 

 of lime will, therefore, lead to a gradual increase in soil acidity. How- 

 ever, nitrates may be found even in very acid soils. This was explained 

 by Hall and associates 41 as due to the fact that, under acid conditions, 

 nitrate formation takes place in films surrounding the small isolated 

 particles of CaC0 3 . The addition of CaC0 3 has, therefore, a decided 

 stimulating effect on nitrate formation, particularly in acid soils. 42 



"Gaarder and Hagem, 1920-1923 (p. 77). 



38 Gerretsen, 1921 (p. 736). 



39 Meek, C. S., and Lipman, C. B. The relation of the reaction and salt 

 concentration of the medium to nitrifying bacteria. Jour. Gen. Physiol., 5: 

 195-204. 1922. 



40 Barthel and Bengtsson, 1920 (p. 528); Waksman, S. A. Microbiological 

 analysis of soils as an index of soil fertility. V. Methods for the study of nitrifi- 

 cation. Soil Sci., 15: 241-260. 1923. 



41 Hall, A. D., Miller, N. H. J., and Gimingham, C. T. Nitrification in acid 

 soils. Proc. Roy. Soc. B., 80: 196-212. 1908. 



42 Fischer, H. Versuche iiber Stickstoffumsetzung in verschiedenen Boden. 

 Landw. Jahrb., 41: 755-822. 1911; Vogel, J. Ammoniak und Salpeterassimila- 

 tion durch Mikroorganismen. Centrbl. Bakt. II, 32: 169-179. 1911; Lemmer- 



