502 SOILS : PROPERTIES AND MANAGEMENT 



men, unless a renewal of the supply is brought about in 

 some way. Natural processes provide for an annual ac- 

 cretion through the washing-down of ammonia and 

 nitrates by rain water from the atmosphere, and through 

 the fixation of free atmospheric nitrogen by bacteria ; but 

 without the frequent use of leguminous crops, the supply 

 could not be maintained. Farm practice of the present 

 day requires the application of nitrogen in some form of 

 manure, and, as the end of the commercial supply of com- 

 bined nitrogen is easily in sight, there is urgent need of 

 discovering a new source. This has been done by com- 

 bining calcium with atmospheric nitrogen in the forms of 

 calcium cyanamide and calcium nitrate. 



423. Cyanamid. The trade name for calcium cyana- 

 mide is " cyanamid " and that name is therefore used 

 in this volume. One process for the production of cyana- 

 mid consists in passing nitrogen into closed retorts con- 

 taining powdered calcium carbide heated to a high tem- 

 perature; the product being calcium cyanamide and free 

 carbon : 



CaC 2 + 2 N = CaCN 2 + C 



The free carbon remains distributed in the cyanamide 

 and gives the fertilizer a black color. The nitrogen re- 

 quired for the process is obtained either by passing air 

 over heated copper, or by the fractional distillation of 

 liquid air. 



The fertilizer, as placed on the market, is a heavy, 

 black powder or granulated material with a somewhat dis- 

 agreeable odor. 



424. Composition of cyanamid. 1 Cyanamid as manu- 



1 Cyanamid is a trade name ; the chemical compound is 

 spelled cyanamide. 



