686 PRINCIPLES OF SOIL MICROBIOLOGY 



peptides, and finally to amino acids and acid amides. The latter, when 

 used completely or incompletely as sources of carbon by microorganisms, 

 liberate ammonia. This is oxidized in normal aerated soil to nitrites 

 and then to nitrates. 37 



Some amino acids are probably split off in the first stages of protein 

 hydrolysis. Some ammonia is also formed long before all of the protein 

 is changed to amino acids; some of the latter are decomposed very 

 readily, while others are more resistant. The amino acids may be 

 directly assimilated by microorganisms or they may form condensa- 

 tion products with carbohydrates. Ammonia and nitrates are the final 

 steps in the transformation of nitrogen in the decomposition of proteins; 

 their determination can serve as a good index of the course of decompo- 

 sition of simple and complex organic nitrogenous compounds in the soil. 

 In the case of some soils rich in organic matter, as in the fertile prairie 

 soils, the transformation of the ammonia into nitrate goes on less rapidly 

 than the formation of ammonia, hence the latter may accumulate. 37 " 1 In 

 the presence of carbohydrates or other carbon compounds, serving as 

 sources of energy for microorganisms, the ammonia and nitrate are 

 rapidly changed again to proteins. 



The nitrogen in stable manure is about evenly divided between the 

 urea and ammonia nitrogen, on the one hand, and protein or complex 

 organic nitrogen, on the other. When manure is composted or added to 

 the soil, the urea and ammonia nitrogen readily changes to nitrate. 

 The second part of the nitrogen has to be first liberated as ammonia 

 and may again be reassimilated by microorganisms in the presence of 

 considerable quantities of straw. Only after the excess of available 

 carbohydrates has been decomposed, is the ammonia liberated and 

 changed to nitrate. 38 



The sum total of ammonia and nitrate formed as a result of decompo- 

 sition of a certain organic substance added to the soil can be used as an 



37 A detailed study of the decomposition of nitrogenous substances in the 

 soil is given by Lathrop, 1917 (p. 474) ; in forest soil by Si'ichting, H. Der Abbau 

 der organischen Stickstoffverbindungen des Waldhumus durch biologische Vor- 

 gange. Ztschr. Pflanzernahr. Di'mgung., 1: 113-154. 1922; Chemical nature 

 of soil organic nitrogen is discussed by Jodidi, S. L. The chemical nature of 

 the organic nitrogen in the soil. Iowa Agr. Exp. Sta. Res. Bui. 1. 1911. 



37a Wyatt, F. A., Ward, A. S., and Newton, J. D. Nitrate production under 

 field conditions in soils of Central Alberta. Sci. Agr., 7: 1-24. 1926. 



38 Bright, J. W., and Conn, H. J. Ammonification of manure in soil. N. Y. 

 Agr. Exp. Sta., Tech. Bui. 67. 1919. 



