THE PLANT AND THE ATMOSPHERE 45 



causing the free nitrogen of the air to enter into combination. 

 The ways at present in which this is accomplished are as 

 follows : 



1 i ) One process is the assimilation of free nitrogen by bacteria 

 in symbiosis with leguminous plants. This method is the most 

 promising for practical agriculture. The energy of the sun is 

 utilized and the nitrogen is converted directly into organic matter. 

 Some bacteria appear to be able to assimilate nitrogen without the 

 aid of plants. 



(2) Another process is the electrical production of 

 nitric acid or nitrates. An electrical discharge is passed 

 through air or through a mixture of nitrogen and 

 oxygen, and the oxides of nitrogen produced thereby are 

 absorbed by water or sodium carbonate or caustic soda. When 

 water is the absorbing medium, nitric acid is produced ; if caustic 

 soda or lime is used, nitrates are produced, which may be used 

 as a fertilizer. Since nitric acid is more valuable than nitrate of 

 soda, nitric acid is made when practicable. This method is 

 practicable only when electricity can be produced cheaply, such 

 as is accomplished by water power. 



(3) Production of Calcium Cyanamid. 1 Air, freed from 

 oxygen by passing over heated metallic copper, is passed into a 

 mixture of calcium carbonate and carbon heated in the electric 

 furnace. The first product is probably calcium carbide: 



2CaCO 3 + 50 = 2CaC, + 3CO 2 . 



This is then converted by absorption of nitrogen into calcium 

 cyanamid : 



CaC 2 + 2N = CaCN 2 + C. 



The product contains 15 to 25 per cent, nitrogen and is used 

 directly as a fertilizer. A cheap source of electrical energy is 

 required. 



Circulation of Nitrogen. The circulation of nitrogen is some- 

 what more complicated than that of carbon. The diagram shows 

 the various processes which it undergoes. 

 1 Bulletin 63, Bureau of Soils. 



