Superphosphates Potash 89 



the percentage of the soluble. The method of chemical 

 analysis which has been adopted by the American Associa- 

 tion of Official Agricultural Chemists recognizes this form, 

 and it is, therefore, determined and included in the " total 

 available" in statements of analysis. In one state, New 

 Jersey, the law requires that the dicalcic form only shall 

 be recognized, and it assumes that the agricultural value 

 of this form is equal to that of the soluble. 



DOUBLE SUPERPHOSPHATES 



In addition to the superphosphates made directly from 

 the various materials mentioned, a special substance, 

 called a "double superphosphate," which may be made by 

 dissolving low-grade phosphates with an excess of dilute 

 sulfuric acid, or those too poor in phosphoric acid to make 

 a high-grade superphosphate. The dissolved phosphoric 

 acid thus obtained, together with the excess of sulfuric 

 acid, are separated from the insoluble materials by filter- 

 ing, which acids, after concentration, are then used for 

 dissolving the better phosphates; and because the acids 

 used for dissolving the phosphates contain phosphoric 

 acid, the content of available phosphoric acid in these 

 products is more than double that contained in the ordinary 

 products. These are mostly manufactured in Europe, and 

 are not used to any extent in this country. They possess 

 the advantage of containing a minimum of impurities and 

 a maximum of phosphoric acid in a soluble form. 



In stating the composition of superphosphates, the three 

 forms of phosphoric acid are all recognized. The sum of 

 the soluble and reverted forms is called the "total avail- 

 able," because these, as already stated, are regarded as 

 immediately useful to the plant. In commercial trans- 



