NITRIFICATION. 59 



a small quantity of soil is added to a solution of nitrite the nitrite soon 

 becomes converted into nitrate, under the influence of the fermenta- 

 tion started by the presence of the soil. This shows that the soil 

 must contain the nitrifying organisms. But the bacteria which 

 are isolated from such soil by ordinary methods showed no power 

 of nitrification. Evidently the nitrifying bacteria cannot be found 

 by the ordinary bacteriological methods. 



The cause of the trouble as well as the secret of successful study 

 was soon learned. In bacteriological studies the common method of 

 isolating bacteria is to get them to grow in culture media made by 

 the bacteriologist. The media commonly used contain a certain 

 amount of organic compounds which serve as food for the bac- 

 teria. But experiment soon showed that the presence of the smallest 

 amount of organic matter is directly injurious to the nitrifying 

 bacteria, so that they will not grow at all in ordinary culture media. 

 It was necessary to devise some culture media that contained no 

 organic matter, and as soon as this was done it was possible to 

 isolate from the soil bacteria having the power, under proper con- 

 ditions, of oxidizing ammonium and nitrite compounds into ni- 

 trates. For a while the results of experiments were in some con- 

 fusion, since in some cases nitrates appeared to be formed, while in 

 others they did not. It became evident that nitrification was not a 

 simple phenomenon, and further study showed that the nitrification, 

 as occurring in ordinary soil, is a two-fold process. The first step in 

 the process oxidizes the ammonia into nitrites. In most of the ex- 

 periments the nitrogen was put into the culture fluids in the form of 

 sulphate or carbonate of ammonia and this was readily oxidized into 

 nitrite. The second step was the oxidation of the nitrites into 

 nitrates. The two steps are not only independent, but they are 

 brought about by two different species of bacteria. One organism 

 has the power of producing nitrite out of ammonia, but can carry 

 the oxidation no farther, failing to produce nitrates. The second 

 species can act upon the nitrites, carrying their oxidation up to the 

 form of nitrates, but it has no power to act upon ammonia. The 

 two together can produce the complete nitrification of both am- 

 monium and nitrite compounds. 



