408 MICROORGANISMS 



465. Interrelations of Soil Bacteria. In this account of the 

 relation of bacteria to soil fertility, we have given only the 

 mere thread of the story. Many interesting interrelations 

 exist between the different kinds of soil bacteria which our 

 limited space will not permit us to consider. 



Some different kinds of bacteria are helpful to one another. 

 In some cases they are almost powerless to carry on their 

 processes without this mutual aid. In other cases, different 

 kinds are seriously detrimental to one another, not only com- 

 peting for food, but through their wastes and secretions hinder- 

 ing one another's development. Some even undo the work that 

 others do. For example, there are great numbers of bacteria 

 in barnyard manure and some of them in the soil generally 

 that have the power of breaking up the nitrates of the soil 

 and setting the nitrogen free in the atmosphere. These are 

 called DENITRIFYING BACTERIA and they tend to undo the work 

 accomplished by the three groups discussed above. 



Some of the soil bacteria must have free gaseous oxygen 

 supplied them for respiratory purposes, while others can, not 

 only get along without free oxygen, but can not carry on their 

 processes in its presence. These latter obtain the oxygen 

 which they use in respiration from chemical compounds which 

 contain it. The kinds of bacteria that must have free oxygen 

 are known as AEROBIC BACTERIA; those that do not require 

 it are known as ANAEROBIC BACTERIA. 



Many other details like these are known to bacteriologists 

 and doubtless there are very many more important relations 

 that exist among these organisms and between them and 

 their environment which have not yet been discovered. 

 Furthermore, other soil elements are affected by bacteria in 

 ways very similar to those in which nitrogen is affected, and 

 numerous complex relations exist among these different 

 elements and among the organisms that affect them. 



Thus you see that the soil is not the dead, inert, and un- 

 changing thing that you might suppose it to be. It is teem- 

 ing with life and endless change. It presents an endless list 



