20 MANUFACTURE OF FERTILIZING MATERIALS 



retention and radiation of heat. In combination 

 with oxygen it forms the chief food of growing 

 plants, the carbon of the carbon dioxide of the 

 air being elaborated into the tissue of the plants 

 and the oxygen returned to the atmosphere. 

 The content of carbon dioxide in the air is from 

 three to five parts per 10,000 by volume. As a 

 constituent of carbonates this element helps to 

 form some of the most important ingredients of 

 the earth's crust, namely, limestone, marbles, 

 dolomites, etc., and, as a result of organic activity, 

 it is found in the shells of the crustaceans. The 

 calcareous matters of the soil, that is, the car- 

 bonates of lime therein found, are of the highest 

 importance from an agricultural point of view. 

 They not only favor the process of converting 

 nitrogenous bodies into forms suitable for plant 

 food, but also exert a most potent influence on the 

 physical state of the soil and its capacity for hold- 

 ing water and permitting its flow to the rootlets of 

 the plant. 



Sulphur occurs in nature in both the free and 

 combined state. In the free state it is found in 

 volcanic regions such as Sicily, Iceland and the 

 Western United States. Its usual form of occur- 

 rence is in combination with the metals to form 

 sulphides, or with oxygen and a metal to form sul- 

 phates. Sulphur and iron combine to form iron 

 pyrites or iron disulphide (FeS2), while sulphur, 

 oxygen, and calcium are found in gypsum, an 

 important fertilizing material. 



