42 AMERICAN MANURES. 



was present in excess, without it they could 

 never come to maturity. 



Since nitrogen is so plentifully furnished in 

 the atmosphere, a superficial observer would 

 suppose that plants would never suffer from a 

 lack of this substance. The closest investiga- 

 tions have failed to show that they can assimilate 

 nitrogen in its pure state from the atmosphere ; 

 on the contrary, some plants discharge the 

 nitrogen that is absorbed by the roots. On the 

 other hand it has been directly proved, by a 

 large amount of evidence, that it enters into 

 their roots, either as ammonia or nitric acid. 



There are various opinions as to the relative 

 value of ammonia and nitric acid, in furnishing 

 nitrogen to plants; but as the application of 

 either substance is followed by direct beneficial 

 results, it may be inferred that they are nearly 

 equally valuable in proportion to the amount of 

 nitrogen contained in each, although it is very 

 probable that ammonia is more directly available. 



Many persons suppose that the atmosphere 

 furnishes a sufficient amount of this substance 

 for the wants of vegetation, and that it is 

 brought down by rains and dews into the soil. 

 That ammonia is continually being formed from 

 decaying animal and vegetable matter, and that 

 it escapes into the atmosphere, we will admit; 

 and we will now examine how far this supply 



