166 Bacteria in Relation to Country Life 



bors, and the latter are enabled thereby to grow 

 normally. 



Denitrifying bacteria. — There are in the soil certain 

 bacteria capable of breaking down nitrates and of 

 returning their nitrogen to the air in the gaseous state. 

 These organisms are called "denitrifying" bacteria. 

 There are other organisms in the soil which possess 

 this power only in a limited degree. Some of them can 

 reduce nitrates to nitrites, or to ammonia; others can 

 reduce nitrites to nitrogen gas. But when two of these 

 species are living together the destruction of the nitrates 

 may be complete, since each performs part of the work 

 the other cannot do. The combined work of the two or 

 more species, which may be referred to as "associative 

 action," is seen, also, in the ammonification of humus- 

 nitrogen. There are species that are incapable of pro- 

 ducing ammonia in the soil, even in slight amounts, 

 yet are capable of developing a marked ammonifying 

 power in the presence of other organisms, or of influenc- 

 ing favorably the ammonifying action of their neigh- 

 bors. 



Influence of bacteria on one another. — There is still 

 much to be learned concerning the influence of the dif- 

 ferent kinds of bacteria on one another. We are still 

 ignorant of the manner in which the associative action 

 is modified by cHmate and the mechanical and chemical 

 constitution of the soil. In the case of ammonification, 

 we know that the transformation of the humus-nitrogen 

 may be rapid or slow, that it may be accompanied 

 by large or slight losses of nitrogen in the gaseous state. 

 We know, also, something of the conditions that hasten 



