288 DISTRIBUTION OF BACTERIA 



The Nitrifying Bacteria. They are exceedingly impor- 

 tant organisms from the standpoint of the cycle of nitrogen 

 in nature. The saprogenic and saprophilic bacteria break 

 down the organic matter to the form of ammonia. This 

 is available to a limited extent, if at all, to the green plants. 

 The conversion of the ammonia formed during the process 

 of putrefaction into the nitrates is a matter of greatest im- 

 portance in soil fertility. This change is effected by the 

 nitrifying bacteria. In order that they may do their work, 

 certain conditions must exist. The soil must be readily 

 permeable to air, since these bacteria are oxidizing organisms. 

 Drainage and methods of cultivation are of very great impor- 

 tance in this connection. In order that nitrification may 

 proceed successfully, it is necessary that the acids produced 

 by them be neutralized by bases. A soil to encourage nitrifi- 

 cation must, then, have suitable bases. The question of soil 

 fertility is then, in its last analysis, a bacteriological problem. 



Denitrifying Bacteria. The denitrifying bacteria are 

 frequently found in the soil. Their action is just the reverse 

 of the nitrifying bacteria ; that is, they reduce the nitrates 

 back to ammonia. In this way the nitrogen is lost as food 

 for the higher plants. Such bacteria are, then, very unde- 

 sirable from the agricultural standpoint. It is very fortu- 

 nate that the conditions which are demanded by the nitrify- 

 ing bacteria are those which are most unfavorable for this 

 class ; hence, conditions which favor nitrification interfere 

 or prevent the activity of the denitrifying bacteria. 



