158 SOILS AND MANURES 



is probably owing to their extreme sensitiveness to des- 

 iccation that they are not found in air. They are always 

 present in water from wells and most other sources, but 

 are, naturally, more numerous in sewage and water con- 

 taining organic impurities. 



Pood. The nitrifying bacteria are adapted for growth 

 in a very dilute medium. They require the usual mineral 

 elements of plant food, viz., sulphates and phosphates of 

 potash, magnesia and lime, but they can assimilate free 

 nitrogen and obtain carbon from carbon dioxide, and are, 

 therefore, independent of organic matter. 



Temperature. The most favourable temperature for the 

 growth of nitrifying organisms is 37 C. Below 5 C. they 

 develop very slowly, and very little nitric acid is, there- 

 fore, formed in the soil during the cold season of the year. 

 The maximum limit is about 50 -- 55 C. ; at higher tem- 

 peratures the organisms are destroyed. 



Neutral Medium. They grow best in neutral or faintly 

 alkaline solutions. The final products are acid, and, unless 

 removed as they are formed, inhibit the action of the 

 organisms. Alkaline hydrates and carbonates obviously 

 cannot be employed to neutralise the acid, but the milder 

 alkalinity of bicarbonates has a less unfavourable effect. 

 Calcium carbonate is, however, by far the most suitable 

 substance for the purpose. It is neutral and insoluble, 

 but readily reacts with the nitric acid forming neutral 

 soluble salts. To promote nitrification in this way is 

 one of the most important functions of lime in the 

 soil (p. 115). 



The Rate of Nitrification. So far as is known all kinds 

 of nitrogenous organic matter undergo nitrification, but 

 some are more susceptible than others. For example, 

 wool, horn and fibrous tissues are much more resistant 

 than blood, urine, cellular fungi, etc. The rate at which 

 nitrification takes place in the soil, provided it is not 



