96 AGRICULTURAL BACTERIOLOGY. 



explained the confusion by a second discovery, showing 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 experiments 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. 

 But Winogradsky showed, not only that the two steps are in- 

 dependent, but that they are brought about by two different 

 species of bacteria. The organism which he first isolated had 

 the power of producing nitrite out of ammonia, but, when he 

 was especially careful to obtain absolutely pure cultures, the 

 further oxidation into nitrates did not occur. The more care- 

 ful his work, the less evidence he obtained for the production 

 of nitrates, and this led him to believe that there must be a 

 second bacterium mixed with the first in some of the experi- 

 ments. Acting upon this suggestion he was finally successful 

 in separating the two species, and obtained a second bacterium 

 which had the power of oxidizing nitrites into nitrates. But 

 this second species had no power of oxidizing ammonia. 

 When the two were together the ammonia was completely 

 oxidized into nitrate by their combined action. His early ex- 

 periments were sometimes performed with such mixtures and 

 the confusion of the results was thus easily explained. Wino- 

 gradsky's conclusions were soon verified, by others so that we 

 now know of at least two types of soil bacteria associated with 

 this phenomenon of nitrification. These bacteria have been 

 called the Nitrobacteria. They are as follows : 



Nitrous Bacteria. Originally called Nitromonas and more re- 

 cently Nitrosomonas. These organisms perform the first step 

 in the nitrification, oxidizing the ammonia compounds into the 

 form of nitrous acid, which forms nitrous salts with the soil 

 alkalies. They cannot^carry the process on to the formation 

 of nitric acid. At least two species are known, one being 



