CONDITIONS OF LIFE OF NITRIFYING ORGANISMS 69 
tion of the ammonia compounds, a process that apparently can 
furnish them with all they need. But whatever its source, these 
nitrifiers are able to live under conditions in which but few other 
organisms can exist. 
Until recently, in fact, it was supposed that no other organisms 
could live under such conditions Lately, however, certain other 
bacteria have been found that can live in the absence of all organic 
matter, obtaining their energy like the nitrifiers from some chem- 
ical change which they produce. These organisms are not of 
great importance in practice; but they are of considerable theoret- 
cal importance, as showing that the nitrifiers are not an excep- 
tion among all other forms of life 
The detrimental effect of organic matter upon the nitrifiers 
has been observed only in laboratory culture media, and does not 
appear to be of much significance in ordinary soil. If organic 
matter had this same effect in soil, no nitrification could occur in 
compost heaps 01 in freshly manured soil. But nitrification takes 
place more vigorously in soil than in solutions, and proper testing 
has shown that the organic matter in soil does not prevent a vigor- 
ous nitrification. It has even been found to take place in manure. 
This difference between soil and culture media is a puzzling one 
and has never been fully explained; but it is probably associated 
in some way with the better aeration in soil. Growing in the 
moisture films of soil, these bacteria have much better access to 
oxygen than in liquid media; and presumably this intimate asso- 
ciation with the oxygen of the soil air overcomes the harmful 
effects of the organic matter. 
Moisture.—The nitrifiers require a moderate amount of 
moisture. Too dry a soil will not allow of their growth. But, on 
the other hand, too much moisture is equally detrimental. It 
has just been stated that they do not grow so readily in laboratory 
solutions asin soil. Itis also a fact that in very wet soil, ‘‘ water- 
logged,” nitrification is greatly reduced or lacking. 
Reaction.—The nitrifiers cannot develop in an acid medium 
