56 SOILS 



legumes being the sole recipients of its favor, and then 

 only through the medium of bacteria. 



While it is impossible to give relative values to essen- 

 tial elements of plants and animals, if it were not impos- 

 sible, nitrogen, undoubtedly, would be given first rank 

 among all life-producing agencies. 



Besides its occurrence in animal and vegetable forms, 

 nitrogen frequently is combined as follows : 



1. With potassium and oxygen, forming the potassium 

 salt known as saltpeter. 



2. With sodium and oxygen, forming the sodium salt 

 known as Chili saltpeter. 



3. With hydrogen, forming ammonia. 



Carbon: the basic element of heat and energy. This 

 element is diffused throughout the world. It is most 

 abundant in combined forms, the exceptions being: min- 

 eral graphite or black lead and the diamond pure crystal- 

 lized carbon. 



We are never interested in carbon as a plant food, since 

 this element enters from the air into the leaves. In plant 

 and animal building, carbon is found in every constituent, 

 hence it becomes a central element in organic processes. 



In inorganic forms, among its many combinations may 

 be mentioned: 



1. With oxygen it forms the carbonic acid gas of the 

 atmosphere, the natural waters, and the limestones. 



2. With oxygen and hydrogen it forms coal. 



3. With hydrogen it forms coal-oil or petroleum. 



All of these substances are associated with combus- 

 tion, showing that plant building is an effort on nature's 

 part for storing heat and energy for future needs of the 

 world. 



Silicon: the sand maker. Silicon is never found in 

 nature except in combination. Its combination with oxy- 



