ORGANISMS THAT REDUCE NITROGEN COMPOUNDS 197 



under conditions which are known to be favourable to the growth 

 of denitrifying bacteria. To obtain a knowledge of these con- 

 ditions it was necessary to study the organisms as pure cultures, 

 and these were not difficult to obtain. The first two that were 

 isolated were named respectively Bacterium denitrificans a and 

 Bacterium denitrificans /3. These were followed by Bacillus denit- 

 rificans, Bacterium dentrificans, and Bacterium Stutzeri. It is 

 interesting to note that Bacterium denitrificans can reduce nitrates 

 to free nitrogen only when in symbiosis with Bacillus coli com- 

 munis or Bac. typhi abdominalis. Of late years several others have 

 been described which need not be considered here. The results of 

 these researches all tend in the same directions with regard to the 

 physiological characteristics of the denitrifying bacteria. The most 

 important of these is the fact that they can thrive only when an 

 abundance of assimilable organic food and a nitrate, e.g. saltpetre, are 

 presented to them together. Another important fact which has been 

 elicited is that though they require the presence of oxygen, yet they 

 thrive best when only a small amount of that gas is present. In the 

 soil these conditions do not generally hold, and the danger from 

 denitrifying bacteria lies in the congested manure heap rather than 

 in the soil. In the latter the conditions are more favourable to the 

 nitrifying bacteria, but in congested manure heaps the aeration is 

 necessarily small, the amount of organic matter large, and if, through 

 ignorance or accident, a good supply of a nitrate, e.g. saltpetre, is 

 added, a great amount of nitrogen loss through denitrification will 

 almost inevitably take place. In practice, however, it very seldom 

 happens that all three factors are combined together, so that loss 

 of nitrogen in this manner is not a source of great anxiety to the 

 agriculturist. 



The denitrifying organisms are widely distributed in nature, for 

 they are found in the air, in water, and in all cultivated soils ; also 

 in the excrement of all herbivorous animals and all animals partly 

 herbivorous and partly carnivorous. They are absent from uncul- 

 tivated soils, and from the excrement of purely carnivorous animals. 



Seeing that the activities of these organisms is of such great 

 practical importance to the agriculturist and manufacturer, it is 

 a pity that their morphological characteristics have been so much 

 neglected. There is still considerable vagueness in the description 

 of the various species of denitrifying bacteria a vagueness which has 

 probably not infrequently led to a particular organism being designated 

 bv different names. 



