338 THE PRINCIPLES OF SOIL MANAGEMENT 



cium chloride or fluoride, converting the bases into sul- 

 fates and freeing carbon dioxide, water, hydrochloric 

 acid and hydrofluoric acid. The resulting superphos- 

 phate is therefore a mixture of monocalcium phosphate, 

 dicalcium phosphate, tricalcium phosphate, calcium 

 sulfate, and iron and aluminum sulfates. 



In the superphosphates made from bone, the iron 

 and aluminum sulfates do not exist in any considerable 

 amounts. 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 fer- 

 tilizer is the same; but the remaining tricalcium phos- 

 phate has a greater value in the bone than in the rock 

 superphosphate. 



The superphosphates made from animal bone con- 

 tain about 12 per cent available phosphoric acid, and 

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

 contain some nitrogen. Bone-ash and bone-black super- 

 phosphates contain practically all of their phosphorus 

 in an available form, but they contain little or no nitro- 

 gen. South Carolina rock superphosphate contains from 

 12 to 14 per cent available phosphoric acid, including 

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

 Florida rock superphosphates contain from 17 per cent 

 downward of available phosphoric acid, part of which 

 is reverted. The Tennessee superphosphates vary from 

 14 to 18 per cent available phosphoric acid. 



202. Reverted phosphoric acid. On standing, a 

 change sometimes occurs in superphosphates by which 

 a part of the phosphoric acid becomes less easily soluble, 

 and to that extent the value of the fertilizer is decreased. 



