65 



Nitric acid, however, can be rendered in- 

 nocuous to the nitrifying organisms by giving 

 it access to a base with which it may unite 

 to form a neutral salt. Whereas nitric acid 

 in its free state is highly corrosive, it becomes 

 perfectly innocuous when it is brought into 

 contact with such substances as lime, potash,, 

 or magnesia, with which it enters into chemical 

 combination to form nitrates of lime, potash, 

 and magnesia respectively, substances which 

 have no prejudicial effect upon living organ- 

 isms with which they may be brought into 

 contact. Nitrate of soda, a manure that 

 is largely sown by hand, is an example 

 of a combination of nitric acid and a 

 base that has no injurious effect on animal' 

 tissues. The base with which nitric acid 

 most frequently unites in the soil is lime > 

 and it is the stimulus given to nitrification 

 that chiefly accounts for the marked effects, 

 of this substance when applied to land rich 

 in humus. 



Not only do organisms form nitrates out 

 of plant and animal remains, but another 

 set of organisms can attack nitrates and 

 reduce them to free nitrogen, which escapes 

 into the air and is lost to crops. This process 

 of denitrification, as it is called, takes place 

 in the absence of a sufficient supply of air^ 

 c 



