FERTILIZERS. 85 



the bones of animals at some period in the ancient his- 

 tory of the earth. Many of these deposits are used as a 

 source of phosphoric acid for fertilizers. Those used in 

 this country are obtained from South Carolina, in what 

 is called " South Carolina rock," and from Canada, in the 

 form of a green mineral, called apatite. 



4. By a process of manufacturing steel there is formed 

 a waste product, or slag, which contains phosphoric acid. 

 This is sometimes used in preparing fertilizers. 



Sources of Potash. 1. Wood ashes contain to some 

 extent all the mineral or inorganic elements of plant 

 food, but are particularly rich in carbonate of potash. 

 They have been used as fertilizers since very ancient 

 times. Until quite recently they have served as the only 

 source of potash for artificial fertilizers. 



2. In the mines of Germany, potash is obtained in 

 the form of salts of several varieties. These are known 

 as " German potash salts." Chloride of potash, com- 

 monly called " muriate " of potash, obtained from this 

 source, is largely used in this country for fertilizing 

 purposes. 



Guano. Certain islands off the western coast of South 

 America have for centuries been the haunt of countless 

 numbers of sea birds. The droppings of these birds con- 

 stitute guano. It has accumulated in some places to 

 the depth of from twenty to fifty feet. The general ab- 

 sence of rain in those regions has prevented the valuable 

 elements from being washed away. Large quantities of 

 this guano are shipped to different parts of the world, 

 and are either applied directly to the soil, or used in pre- 

 paring other commercial fertilizers. 



Prepared Fertilizers. The various substances enumer- 

 ated may be used singly as fertilizers, or in combination. 



