198 THE FIRST BOOK OF FARMING 



available phosphoric acid and two to three per cent, 

 of nitrogen. 



Dissolved Bone Black. Bone charcoal is used for 

 refining sugar. It is then turned over to the ferti- 

 lizer manufacturers who sell it as " Bone Black " or 

 treat it with sulphuric acid and then put it on the 

 market as dissolved bone black. 



The bone black contains thirty to thirty-six per 

 cent, of insoluble phosphoric acid. . 



The dissolved bone black contains 15 to 17 per 

 cent, of available phosphoric acid and I to 2 per 

 cent, insoluble. 



" Thomas Slag," " Phosphate Slag," " Odorless 

 Phosphate." Phosphorous is an impurity in certain 

 iron ores. In the manufacture of Bessemer steel 

 this is extracted by the use of lime which melts in 

 the furnace, unites with the phosphorous and brings 

 it away in the slag. This slag is ground to a fine 

 powder and used as a fertilizer. It contains II to 

 23 per cent, of phosphoric acid, most of which is 

 available. 



Superphosphate. The term superphosphate is ap- 

 plied to the phosphates that have been treated with 

 sulphuric acid to make the phosphoric acid avail- 

 able. Dissolved bone, dissolved bone black, and the 

 dissolved phosphate rocks are superphosphates. 



Fish Scrap, mentioned as a source of nitrogen, is 

 also a valuable source of phosphoric acid, contain- 

 ing 6 to 8 per cent., which is quite readily available 

 owing to the rapid decay of the scrap. 



