COMMERCIAL FERTILIZERS 515 



quantities. However, as long as the phosphorus remains 

 in the form of monocalcium phosphate, the value of a 

 pound of available phosphorus in the two kinds of fertilizer 

 is the same ; but the remaining tricalcium phosphate has 

 a greater value in the bone than in the rock superphosphate. 



The superphosphates made from animal bone contain 

 about 12 per cent of available phosphoric acid and from 

 3 to 4 per cent of insoluble phosphoric acid. They also 

 contain some nitrogen. Bone ash and bone black super- 

 phosphates contain practically all their phosphorus in an 

 available form, but they contain little or no nitrogen. 

 South Carolina rock superphosphate contains from 12 to 

 14 per cent of available phosphoric acid, including from 

 1 to 3 per cent of reverted phosphoric acid. The best 

 Florida rock superphosphates contain from 17 per cent 

 downward of available phosphoric acid, some of which is 

 reverted. The Tennessee superphosphates contain from 

 14 to 18 per cent of available phosphoric acid. 



Double superphosphates, — In making superphosphates 

 a material rich in phosphorus must be used, not less than 

 60 per cent of tricalcium phosphate being necessary for 

 their profitable production. The poorer materials are 

 sometimes used in making what is known as double super- 

 phosphates. For this purpose they are treated with an 

 excess of dilute sulfuric acid; the dissolved phosphorus 

 and the excess of sulfuric acid are separated from the mass 

 by filtering, and are then used for treating phosphates 

 rich in tricalcium phosphate and thus forming superphos- 

 phates. The superphosphates so formed contain more 

 than twice as much phosphorus as those made in the 

 ordinary way. 



433. Reverted phosphoric acid. — A change sometimes 

 occurs in superphosphates on standing by which some of 



