Superphosphates Potash 91 



from bone-black, bone-ash or mineral phosphates, and the 

 variation is due both to the variability of the raw materials 

 and the difficulties involved in their change into super- 

 phosphates. The usual guarantee on an animal bone 

 superphosphate is 12 per cent available, and from 3 to 5 

 per cent of insoluble. These superphosphates also differ 

 from the mineral superphosphates in containing nitrogen 

 in addition to their phosphoric acid. They are, therefore, 

 really ammoniated superphosphates. 



Well-made superphosphates contain no free acid. 



In the earlier history of the use of acid phosphates, or 

 rock superphosphates, objections were urged against 

 them, and are to some extent at the present time, because 

 of the supposed deleterious effects of the acids contained in 

 them, and these objections were undoubtedly encouraged, 

 certainly not discouraged, by those manufacturers 

 who used only genuine bone superphosphates. While the 

 objections on this ground may have had some basis in 

 earlier times, before their manufacture was well under- 

 stood, there can be no rational objection to their use at the 

 present time, when they are properly made ; for while in 

 fresh superphosphates a portion of the phosphoric acid 

 may be in the form of "free" phosphoric acid, this form in 

 ordinary superphosphates is practically all combined with 

 lime or other minerals before it is placed upon the market, 

 and there is really no more "free" acid in the rock super- 

 phosphate than in any other. It is quite likely this 

 erroneous impression arose from the fact that strong sul- 

 furic acid was used in the manufacture, and the belief 

 existed that it remained as such. No free sulfuric acid 

 exists in well-made superphosphates. The sulfuric acid is 

 combined with the lime to form gypsum, as already de- 



