The Soil and They That Dwell Therein 



273 



are concerned, and man must purchase costly fertilizers to re- 

 plenish the nitrogen lost through the harvesting of crops. Manu- 

 facturing processes have been devised, however, for the extraction 

 of atmospheric nitrogen. The Haber Process, for example, forces 

 the combination of nitrogen and hydrogen at about 200° C. under 



Fig. 49. Azotobacter chroococcum. {From Soil Microbiology, S. A. 

 Waksman. Copyright 1953, John Wiley and Sons, Inc., New York.) 



SL pressure of 200 atmospheres ( an atmosphere is about 14.7 pounds 

 per square inch at sea level) in the presence of iron particles 

 which serve as a catalyst. Ammonia is formed by this chemical 

 union, and the ammonia in turn is converted into ammonium salts, 

 several millions tons of which are produced annually in the United 

 States. 



THE CARBON CYCLE 

 Carbon constitutes another primary element in all protoplasm, 

 and thus carbon is an essential substance in nature. Althou2;h the 



