NITRATES AND NITRITES 155 



useful also. Mixtures of these with glucose, starch, and 

 other carbohydrates are utilized also, but not usually the 

 latter alone. Many of the alcohols, such as ethyl, propyl, 

 and isobutyl alcohols and glycerine are oxidized by them, 

 and hippuric and uric acids are utilized also by some 

 species. Many organic compounds, which are not found 

 to be of service to pure cultures of denitrifying bacteria, 

 are indirectly utilized when the latter are associated with 

 putrefactive organisms. A number of the compounds 

 so readily utilized by denitrifiers are produced by the 

 action of putrefactive organisms upon the complex in- 

 soluble compounds in animal and vegetable tissues, 

 humus and the undigested materials in faeces. Some 

 organisms, such HS Bact. denitrificans, L. and N., are known 

 which can set free nitrogen from nitrites ; they are 

 unable to attack nitrates directly, but when grown sym- 

 biotically or along with B, colt and other species which 

 are able to carry out for them the preliminary reduction 

 of the nitrate to nitrite, free nitrogen is evolved. In 

 manure and compost heaps there is little or no denitri- 

 fication with its consequent loss of nitrogen in the 

 gaseous form, since the necessary nitrates are generally 

 missing, the excess of soluble organic matter and absence 

 of aeration preventing their formation except in the upper 

 layers of the heaps. In well-aerated composts, however, 

 and in farmyard manure loosely put together nitrification 

 may occur and be followed by denitrification when the 

 heaps become saturated with water through exposure to 

 heavy rains. In ordinary arable soils loss of nitrogen 

 by denitrification is uncommon, for where nitrates are 

 abundant the easily assimilated carbon compounds upon 

 which denitrifiers depend are wanting. -Tfiat it may 

 occur in poorly aerated, water-logged soils under anaerobic 



