104 BACTERIA AND THE NITROGEN CYCLE 



rivers, in all the sewers and refuse heaps of human communities, the two 

 classes of micro-organisms are incessantly at work, sometimes alone, some- 

 times together, and the greater part of all organic nitrogen is sooner or later 

 liberated in the form of ammonia, together with a small amount of free 

 nitrogen*. The free nitrogen is at once utilisable by the root-nodule 

 organisms, and by other bacteria in the soil. The ammonia nitrogen, on the 

 other hand, must be ' mineralized ' before it can be used by green plants, 

 must be oxidized and combined with a base to form a nitric salt f. This 

 change, known as nitrification, was formerly thought to be a purely chemical 

 phenomenon, the oxidation being presumably effected by the free oxygen 

 of the atmosphere. This view is now known to be incorrect, evidence 

 having gradually accumulated which proves irrefutably the biochemical 

 nature of the process. The bacteria to whose activity it is due defied for 

 many years all attempts at isolation, until Winogradsky, the Russian in- 

 vestigator, succeeded in separating them from the soil and obtaining pure 

 cultures (72). His researches too brought to light the unique fact that the 

 nitrifying bacteria are absolutely prototrophic in their mode of life and 

 we owe to him glimpses into the conditions of existence among the lowest 

 organisms. 



The nitrifying bacteria are everywhere present in the soil of our 

 gardens, fields and meadows, and in the virgin earth of the mountain sides 

 and plains, untiringly at work preparing the food for plants. They have 

 been cultivated by us unconsciously for centuries in the ' saltpetre beds,' 

 where putrescible material (manure and other animal remains skins, horns, 

 glue &c.) is mixed with lime and allowed to ferment, and there can be 

 little doubt that the Chilian nitre owes its origin to the action of nitrifying 

 bacteria in a recent geological period. It probably arose from the washing 

 down of saltpetre that had accumulated around putrefying organisms in the 

 rainless regions of the coast. 



The isolation of the nitrifying bacteria is not difficult ; but rather 

 different methods are necessary from those used for other bacteria. Highly 

 nutritious culture media are useless, for the nitrifying bacteria are proto- 

 trophic in the strictest sense of the word. They may be cultivated en 

 masse by inoculating with a little garden earth the following solution : 



Grammes. 



Water 1000- 



Potassium phosphate (dibasic) . . 2 



Magnesium sulphate .... -3 



Soda (or MgCO 3 ) .... -5 



Sodium chloride -5 



* The nitrogen of crop residues and of the ploughed-in fallow undergoes just the same changes, 

 t This applies of course also to the sulphate of ammonia from gas-works, so largely used as 



