BACTERIA AND THEIR ROLE IN NATURE 1 9 



often detect their activity from the odor of ammonia coming 

 from the stables or manure piles. 



However, most plants cannot use nitrogen in the form of am- 

 monia; it must be changed into nitrates. This is done by bacteria. 

 One groups feeds upon ammonia and manufactures nitrous acid. 

 Nitrites are poison to plants; hence, if it were left at this stage we 

 would have no plants. Another class of bacteria feed upon the 

 nitrites as fast as they are formed and yield nitric acid. This re- 

 acts with the various minerals of the soil. It is now ready to be 

 taken up by the growing plant and manufactured into nourishing 

 food, beautiful flowers, delicious fruit, or fragrant perfumes 

 for the human family. 



So far only the plant- food already in the soil and the changes 

 through which it passes have been considered. The farmer, how- 

 ever, is more concerned with the substances his soil lacks and 

 which must be added in order to get good crops. In many cases 

 the lacking element is nitrogen. One notes from the fertilizer 

 quotations that this element will cost fifteen cents a pound or over 

 if purchased in the form of sodium nitrate, ammonium sulfate, 

 or dried blood. On making a simple calculation we find that at 

 this price it would cost fifteen dollars for enough to produce 100 

 bushels of corn; eleven dollars for enough to produce 50 bushels 

 of wheat, and seven dollars and fifty cents for enough to pro- 

 duce one ton of alfalfa hay. 



In these calculations it has been assumed that one recovers in 

 the form of corn, wheat, or alfalfa every pound of commercial 

 nitrogen which has been applied, which on the face of it can be 

 seen to be an utter impossibility. Hence, we have to look else- 

 where to obtain nitrogen for our growing crop. Here also bacteria 

 come to the rescue. 



There are seventy-five million pounds of atmospheric nitrogen 

 resting upon every acre of land. None of the higher plants, how- 

 ever, have the power of taking this directly out of the air. One 

 family of plants, the Leguminosze, in which are included peas, 

 beans, alfalfa, clover, and many others, if properly infected by 

 bacteria, have the power of using this atmospheric nitrogen. Un- 



