58 NITRIFICATION AND DEN ITRIFI CATION. 



from nitrates. Nitrate formation is, then, a matter of the greatest 

 significance. While there are, perhaps, some other methods by 

 which nitric acid can be formed, beyond doubt the nitrate store in 

 the soil has been formed chiefly through the process of nitrification. 



Nitrification in the Soil. It is very easy to demonstrate that 

 such nitrification actually takes place in ordinary soil. If we place 

 a quantity of soil in a proper vessel and subject it at intervals to a 

 chemical analysis, it will be found that there is an increase in the 

 amount of nitrates present, after it has remained undisturbed for a 

 few weeks. This fact has been known for nearly a century. The 

 next step in the discoveries was made in 1877, when it was demon- 

 strated that this nitrification is associated with the presence of living 

 matter in the soil. This can be proved by placing two lots of the 

 same soil under such conditions that in the one phenomena of life 

 may go on, while in the other they are stopped. If, for example, one 

 lot of soil is sterilized by heating it sufficiently to destroy the living 

 germs present, and then this soil is compared with another lot 

 treated in all respects the same, except that it is not sterilized, the 

 latter will be found to increase its nitrates, while the former will show 

 no such increase. The same results are obtained if the soil is mixed 

 with antiseptics which prevent bacteria growth. In short, any- 

 thing which prevents the occurrence of life phenomena in the soil, 

 prevents the nitrification. 



Isolation of the Nitrifying Organisms. Such experiments 

 repeated many times and verified by numerous observers demon- 

 strated that nitrification is the result of a living process. Inasmuch 

 as such soil contains no plants large enough to be seen, it follows that 

 the living agent of nitrification must be some form of microorganism. 

 It proved, however, to be a very difficult matter to find the organisms 

 concerned in the process. The number of bacteria in the soil is large 

 and many different species are there found. But although many 

 of these bacteria were isolated and carefully tested, for a long time 

 none proved to have any power of nitrification. Most of them, indeed, 

 produced the reverse effect, that of deoxidizing nitrates, but none 

 of them raised the nitrites into a state of nitric acid. None of them 

 could oxidize ammonia so as to form nitric or even nitrous acid. If 



