REACTION OF THE MEDIA 241 



nitrates to nitrites, those capable of reducing nitrates to ammonia 

 or of setting nitrogen free are not very numerous. Severin isolated 

 32 different organisms from horse manure and studied 29 of these. 

 Of this number eight species were capable of complete reduction of 

 nitrates, provided the nitrate concentration be not too great. Nine 

 of the other species were able to reduce nitrates to nitrites. 



Stoklasa divides the denitrifying organisms present in soils and 

 manures into two principal groups. The first group contains 

 Clostridium gelatinosum, Proteus citlc/ttris, P. zenkeri, B. ramosus 

 n. liquefaciens, B. mycoides, B. megatherium, B. subtilis, and B. 

 prodigiosus, and others. The characteristic of these organisms is 

 that they reduce nitrates to ammonia without the formation of 

 elementary nitrogen. The second group contains Bac. hartlebi, 

 B. fluorescens liquefaciens, B. pyocyaneum, B. stutzeri, B. filefaciens, 

 B. nitrovorum, B. centropunctatum, B. denitrificans, B. coli communis, 

 B. typhi-abdoininalis, and others. These organisms as a rule reduce 

 nitrates to elementary nitrogen. 



Beer yeasts (Laurent), especially those of Duclaux, reduce 

 nitrates at 20 C. PeniciUiini glaucum, mucor racemosus, and similar 

 organisms also have a reducing power. 



It is, therefore, true that whereas active nitrogen fixation is a 

 characteristic possessed by only a limited number of microorganisms, 

 the opposite denitrification appears to be a characteristic pos- 

 sessed by many widely dissimilar organisms. 



Reaction of the Media. The denitrifying organisms are similar 

 to the nitrogen-fixing organisms in that they require a slightly 

 alkaline medium in which to function. Von Caron considers that 

 with a sugar concentration of more than 1 or 2 per cent., a depres- 

 sion of denitrification occurs. This probably is due to the formation 

 of fatty acids by the butyric acid ferments of the soil. When it 

 first became known that denitrification may take place in manure 

 heaps, the practice became prevalent to add to the manure sulphuric 

 acid to prevent denitrification. It was found that sulphuric acid is 

 extremely active in preventing denitrification and 0.17 per cent, in 

 the cultural medium was sufficient to prevent the development of 

 the denitrifying organisms. 



Ampola and Garino found that the addition of ground peat 

 showing an acidity of 9.85 per cent, checked the activity of the 

 denitrifying organisms as well as that of other ferments. The 

 organisms, however, were not killed and commenced their activity 

 again as soon as the acidity was neutralized. The soil conditions 

 are favorable to the neutralization of the acid of the peat, and thus 

 the restraining effect of the latter on the denitrifying organisms is 

 nullified. Moreover, an acid condition which would restrain deni- 

 trifiers in soil retards the other beneficial bacteria and higher plants. 

 Hence, while acids may be used at times with some success on 

 manures, it is. not necessary nor practical to add it to soils, 

 16 



