32 BACTERIOLOGY FOR NURSES 



tation would be robbed of its supply of carbon and 

 nitrogen, and plant life would be speedily ended, 

 which in turn would deprive the animal kingdom of 

 food and thus life would no longer be possible. 



A highly important group of saprophytes com- 

 prises the nitrifying and denitrifying bacteria which 

 play so important a part in renewing the fertility 

 of the soil. 



In the process of decomposition which goes on 

 through bacterial activity, there is a loss of consider- 

 able nitrogen into the air where it is no longer avail- 

 able for plant food ; there is in consequence a 

 constant diminishing of nitrogen food for vegetation. 

 Also there is a loss of plant food by drainage into 

 streams and the sea, which, if continued without 

 check, would so deplete the soil of nitrogen com- 

 pounds that plants could no longer live. 



It is this loss of nitrogen from the soil which forces 

 the farmer to apply fertilizers of various kinds to 

 renew the exhausted soil. 



It has been discovered, however, that the presence 

 of certain bacteria in the soil insures a slow but sure 

 gain in the amount of nitrogen compounds, and 

 sterile fields are inoculated by the application of 

 soil containing these bacteria, which are of two or 

 three species working together. The exact process 



