404 MICROORGANISMS 



460. Some Classes of Soil Bacteria. Now, it is probably 

 evident to you why we are so greatly concerned about the 

 supply of nitrogen in the soil in a form available to the green 

 plants. With this thought in mind, you will doubtless be 

 interested to know the part that the soil bacteria play in keep- 

 ing up this supply of available nitrogen in the soil. The 

 bacteria which do this work may conveniently be divided into 

 three groups: (1) The nitrifying bacteria, (2) the nitrogen 

 fixing bacteria, and (3) the nodule bacteria. 



461. The Nitrifying Bacteria. The NITRIFYING BACTERIA 

 transform the various nitrogen-bearing compounds of the soil 

 humus into the nitrate form. This is not a simple process, as 

 you might suppose, but consists of three distinct steps. Each 

 step in the process is "performed by a different set of bacteria, 

 but several different species of bacteria are known for each 

 step. 



The first step in the process is the changing of the proteins 

 and other nitrogen-bearing compounds of the humus into 

 ammonia (Chap. Ill, Art. 194). If you turn over a rapidly 

 decaying mass of organic matter, you can usually detect the 

 characteristic odor of ammonia along with various other 

 odors. 



The second step in the process is carried on by an entirely 

 different set of bacteria and results in changing the ammonia 

 to NITRITES. Nitrites differ, chemically, from nitrates in 

 having relatively less oxygen in their composition. 



The process is completed by another set of bacteria which 

 changes the NITRITES to NITRATES. 



It is obvious that these bacteria which simply transform 

 the nitrogen-bearing compounds of the soil humus into a form 

 available for the higher plants do not actually add any nitrogen 

 to the soil. They only transform nitrogen that is already 

 there. If you recall the fact that the original rock of the earth 

 does not contain any fixed nitrogen, you will see that we, 

 so far, have no means of accounting for the origin of the present 

 nitrogen content of the soil. The bacteria of this group only 



