528 PRINCIPLES OF SOIL MICROBIOLOGY 



nium salts and from amino acids are compared, the latter is found to 

 take place more slowly. 33 This is probably due to the fact that the 

 amino-acids have to be changed first to ammonia and also to the fact 

 that some of the nitrogen will be stored away in the microbial cells 

 which use the carbon of the amino compounds as a source of energy. 



According to Barthel and Bengtsson 34 only the ammonia (and the 

 urea, which is rapidly transformed into ammonia) of stable manure 

 is readily nitrified; the other part of the nitrogen, present in the manure 

 in the form of complex proteins, nitrifies only very slowly since it has 

 to be decomposed first by various microorganisms, with the liberation 

 of ammonia. Nitrite bacteria find conditions in stable manure very 

 favorable for their development so long as aeration is favorable and 

 the manure is not saturated with urine. 35 Deep layers as well as 

 compact manure contain only few nitrifying bacteria, since conditions 

 are not very favorable for their development, under anaerobic or semi- 

 anaerobic conditions. The nitrite bacteria cannot develop in urine, 

 probably because of the presence of some injurious substances and the 

 high concentration of the soluble organic matter; they develop readily 

 in the solid portion of the manure. The free ammonia, especially in a 

 medium of a high alkalinity, is also injurious to the nitrate-forming 

 bacteria. 



Influence of reaction on nitrate formation. The optimum reaction for 

 the respiration of the nitrite-forming bacteria was found 36 to be at pH 

 8.4 to 8.8, with limiting reactions at pH 7.6 and 9.3. The optimum 

 reaction for the respiration of the nitrate-forming bacteria was found 

 to be at pH 8.3 to 9.3 and the limits at pH 5.6 and 10.3. The presence 

 of NaHC0 3 , which acts as buffer at pH 8.4, is, therefore, beneficial to the 

 activities of these organisms, as shown in figure 30. This is true, 

 however, only for the respiration of the organisms but not for their 

 growth. 



The optimum reaction for the growth of the nitrite bacteria is at 

 pH 7.8, with a minimum at pH 7.0 and maximum at pH 8.6; for nitrate 



33 Batham, H. N. Nitrification in soils. Soil Sci., 20: 337-351. 1925. 



34 Barthel, Chr., and Bengtsson, N. The nitrification of stable manure nitro- 

 gen in cultivated soils. Medd. Centralanst. Forsokov. Jordbruksomradet, No. 

 211, 1920; also 1926 (p. 448). 



36 Niklewski, B. Uber die Bedingungen der Nitrifikation im Stallmist. 

 Centrbl. Bakt. II, 26: 388^42. 1910; Diminution du taux de l'azote dans le 

 fumier sous l'influence des bacte>ies nitrifiantes. Bull. Soc. Chim. biol., 6: 

 491-500. 1923. 



"Meyerhof, 1916 (p. 390). 



