552 PRINCIPLES OF SOIL MICROBIOLOGY 



even less so, 39 due probably to a slow availability of the more insoluble 

 constituents, since cellulose is not used to any extent as a source 

 of energy by these organisms. 40 A combination of an available 

 source of energy and anaerobic conditions in the soil (either brought 

 about by high moisture content or replacement of oxygen by hydrogen 

 and C0 2 ) leads to most active denitrification. An increased nitrate 

 content of a soil favoring conditions of complete denitrification will 

 favor the process further. The addition of large quantities of straw 

 manure and green manure 41 has, therefore, an important influence upon 

 the disappearance of nitrates, both through transformation into pro- 

 tein and reduction to nitrites, ammonia and nitrogen gas. It has been 

 found that even difficultly decomposable organic substances may also 

 have a favorable influence upon denitrification in the soil. 42 



The fact that denitrifying bacteria are favored by an alkaline reac- 

 tion and are injured by acids suggests the use of substances which would 

 make the soil reaction acid. However, the final reaction should not be 

 acid enough so as to injure the activities of useful bacteria, like the nitri- 

 fying organisms. Applications of acid phosphate were found to be 

 useful in the preservation of the manure by neutralizing the ammonia ; 

 this brings about a change in reaction and tends, therefore, to lessen 

 denitrification. Of the various nitrates, salts of alkalies and alkali 

 earths are readily denitrified; the reduction of A1(N0 3 )3 is doubtful. 43 

 Iron, manganese, thorium, ythrium and silver nitrates give negative 

 results, due to the toxic action of the cations. Ethyl nitrate is 

 reduced, but not nitro-methane. The same bacteria that reduce ni- 

 trates were found capable of reducing potassium chlorate, arsenate and 

 potassium ferricyanide. 



Importance of nitrate reduction in the soil. Gayon and Dupetit and 

 Deherain and Maquenne established in 1882 that there are bacteria in 

 the soil capable of reducing nitrates to atmospheric nitrogen and oxides 



39 Caron, H. V. Untersuchungen iiber die Physiologie denitrifizierender 

 Bakterien. Centrbl. Bakt. II, 33: 62-116. 1912. 



40 Wright, C. R. The influence of certain organic materials upon the trans- 

 formation of soil nitrogen. Centrbl. Bakt. II, 46: 74-79, 1916; Albrecht, 1922- 

 1926 (p. 517). 



41 Ferguson, M., and Fred, E. B. Denitrification: the effect of fresh and well- 

 rotted manure on plant growth. Va. Agr. Exp. Sta. Rep. 1908, 134-149. 



42 Nolte, O. t)ber Denitrifikation bei Gegenwart von schwerzersetzlichen 

 organischen Substanzen. Centrbl. Bakt. II, 49: 182-184. 1919. 



43 Ampola, G., and Ulpiani, C. Sull'azione riduttrice dei batteri denitrificanti. 

 Gazz. chim. ital. Rome, 29, pt. 1: 49-72. 1899; 34, pt. 2: 301-315. 1904. 



