NITRIFICATION 35 



upon and ammonium salts are formed. According to 

 Emile Marchal, ammonisation takes place essentially 

 under the influence of microbes living in the upper layers 

 of the soil. The Bacillus mycoidcs is one of the most 

 energetic of these, and seems to play a double role, being 

 ammonising in the presence both of nitrogenous organic 

 substances and of nitrates. Urea is ammonised especially 

 by the Micrococcus urece. 



II. Nitrosation. The ammoniacal salts are next con- 

 verted into nitrites. The nitrous organisms can probably 

 attack nitrogenous organic substances such as asparagine 

 and milk, but only feebly, milk being much more rapidly 

 nitrified when the nitrous organisms are mixed with other 

 species. The organisms bringing about this change are 

 short, stumpy, motile bacilli with single polar flagella 

 which are grouped under the generic name of Pseudo 

 monas. 



III. Nitratation. These nitrites are then converted 

 into nitrates. The " nitric " organisms are minute non- 

 motile bacilli known as Nitrobacter. 



Stages II and III are brought about by different 

 species, the nitric organisms having no effect whatever 

 on ammonia, but acting only after this has been oxidised 

 into nitrous acid by the nitrous forms. 



The discovery of Munro that organisms will grow in purely 

 inorganic solutions has been made use of for the isolation of the 

 different species. Solutions such as the following have been used : 



For the Nitrous Organisms. For the Nitric Organisms. 



Ammonium chloride, 0-5 grm. Potassium nitrite, 0-3 grm. 



Potassium phosphate, 0-1 grm. Potassium phosphate, 0-1 grm. 



Magnesium sulphate, 0-02 grm. Magnesium sulphate, 0-05 grm. 



Calcium chloride, 0-01 grm. Calcium carbonate, 5 grm. 



Calcium carbonate, 5 grm. Distilled water, 1,000 c.c. 

 Distilled water, 1,000 c.c. 



These are seeded with traces of earth, and by carrying on the 

 cultivation for many generations a large number of organisms are 



32 



