92 FIRST PRINCIPLES OF AGRICULTURE. 



composition and character of the phosphates from which 

 they are made. Those made from organic phosphates, as 

 bone black and bone ash, are richer in soluble phosphoric 

 acid than those made from animal bone or from mineral 

 phosphates ; since these materials are of such a character 

 as to enable the manufacturer to add sufficient sulphuric 

 acid to convert all of the phosphate present into a soluble 

 form, and at the same time to secure a dry fine product, 

 which is an important consideration in making super- 

 phosphates. 



Mineral phosphates, both because of their hardness and 

 of the presence of other minerals which are attacked by 

 the acid, are less easily dissolved, and require more acid 

 in proportion to the phosphate present than those from 

 organic sources. They are also less absorbent, hence it is 

 more difficult to secure good condition when sufficient acid 

 is used to dissolve all the phosphate. In making super- 

 phosphates from these materials, less acid is used than 

 is required to completely dissolve the phosphates; and 

 there is, therefore, always present in them more or less 

 of the insoluble phosphoric acid. 



In the case of animal bone, too, less sulphuric acid is 

 used than is required to completely dissolve the phosphoric 

 acid; otherwise, a gummy, sticky product would result, 

 due largely to the organic matter in the bone. The in- 

 soluble phosphoric acid in bone, bone-black, and bone-ash 

 superphosphates is, however, of greater value than the 

 insoluble in the mineral phosphates, for reasons already 

 given. 



In superphosphates, too, there is nearly always present 

 a greater or less amount — depending upon the material — 



