194 OUTLINES OF BACTERIOLOGY 



In the case of the nitrate-bacteria we get an illustration of an 

 exception to the general rule that the waste products of an organism 

 tend to retard its further development, for the nitrates which are the 

 chief products of their activity are not injurious to the nitrate-bacteria. 

 Hence, if the conditions are otherwise favourable, they can transform 

 into nitrates the whole of the nitrites presented to them. 



Finally, the nitrate-bacteria are very strongly aerobic, and conse- 

 quently all the special contrivances to secure an abundant supply of 

 oxygen mentioned in connection with the nitrite-bacteria apply equally 

 to these organisms. 



7. NITRIFICATION IN NATURE. 



By nitrification is meant the gradual change of the ammonium salts 

 of the soil into the corresponding nitrate salts. 



The effect of this process is well exhibited in the saltpetre beds of 

 South America, in places where sewage matter is being disposed of, and 

 generally in all places where the exertions of the saprophytic bacteria 

 have resulted in the production of ammonium compounds. As is to 

 be expected from such strongly aerobic organisms, nitrite- and nitrate- 

 bacteria are to be found only on and near the surface. Consequently 

 we find them in greatest number in the first six inches of soil; below 

 this depth their numbers rapidly decrease, and at a depth of two feet 

 they are altogether absent. As they are very sensitive to drought, 

 they are altogether absent from soils that have been dried up for 

 some time. For experimental purposes, therefore, care is taken to 

 use soil that is still somewhat moist. Forest soil is destitute of these 

 bacteria : this is probably accounted for by the inhibitory effects, 

 produced by the presence of a large amount of organic matter which is 

 generally found in such soils. A chemical analysis of a soil that is 

 undergoing nitrification shows no trace, or only small traces, of nitrites, 

 and until this was explained doubts existed in the minds of many as to 

 the validity of the bacteriological explanation of nitrification. A set 

 of experiments, instituted by Omelianski, when supported by all the 

 investigations which have been described in the foregoing paragraphs, 

 put all these doubts to rest. Into a dilute bouillon culture fluid the 

 following combination was introduced : Bac. ramosus (A), an ammonia 

 producer, + a nitrite-producing microbe (B\ + a nitrate-producing 

 microbe (C). After 3-4 days ammonia was found to have been 

 formed ; after 7 days a nitrite reaction was demonstrated, and a 



