140 NITRIFICATION 



of ammonium compounds ; in the soil it is usually 

 supplied by calcium carbonate. Where lime is deficient 

 a dressing of chalk or quicklime must be added before 

 the nitrogen present can appear as a nitrate and be 

 made available for the nutrition of crops. Ammonium 

 salts of organic acids undergo as rapid nitrification as 

 ammonium sulphate or chloride when a basic carbonate 

 is available. The necessity for a basic carbonate of the 

 type mentioned, which reacts with ammonium salts to 

 form ammonium carbonate, suggests that the latter is 

 the only compound which can be directly nitrified. 



Godlewsky showed that the nitrite bacteria obtain the 

 carbon which jwhich they need for the synthesis of those 

 carbon compounds used in their growth from free carbon 

 dioxide gas ; possibly the same holds true for the nitrate 

 forms also. When grown in the presence of insoluble 

 carbonates the organisms do not develop if the atmo- 

 spheric air supplied to them is first passed through a 

 solution of caustic potash, which absorbs carbon dioxide. 

 It is probable that the carbon dioxide of the air is 

 necessary at first, but later the CO 2 of the 

 carbonate present may be sufficient. They carry on 

 their work best in the dark, bright light being injurious 

 to them. As previously mentioned, soluble organic 

 substances act deleteriously upon both species of nitri- 

 fying organisms, the nitrite bacteria being more sensitive 

 in this respect than the nitrate - formers, especially 

 towards nitrogenous substances, such as asparagin or 

 peptone ; it is not possible to cultivate either of them 

 in bouillon or solutions of peptone, sugar, and other com- 

 pounds, which serve well for the growth of most of the 

 ordinary kinds of bacteria. These compounds act upon 

 the nitrifying organisms as phenol, salicylic acid, and other 



