ARTIFICIAL AND CONCENTRATED MANURES. 93 



of the second form of phosphoric acid, the dicalcic, also 

 called "reverted" or "retrograde." This form exists in 

 the greatest amounts in those made from mineral phos- 

 phates, which is believed to be due either to the soluble 

 acting upon the insoluble portions, or to the presence of 

 oxide of iron and alumina, which combine with a portion 

 of the soluble phosphoric acid. The soluble goes back to 

 a less soluble form. 



In stating the composition of superphosphates, the three 

 forms of phosphoric acid are all recognized. The sum of 

 the soluble and reverted is called the "total available," 

 because these forms are regarded as immediately useful 

 to the plant. 



Bone ash and bone black contain on the average sixteen 

 per cent of total available phosphoric acid, practically all 

 soluble ; while those from mineral sources usually contain 

 less than fourteen per cent total available, which includes 

 one to three per cent of dicalcic or reverted. These also 

 contain from one to three per cent of insoluble phosphoric 

 acid. 



Superphosphates made from animal bone are more vari- 

 able in their composition than those made from bone 

 black or the mineral phosphates; this being due largely 

 to the variability of the raw materials, chiefly in respect 

 to the content of phosphoric acid. These differ, too, from 

 the others mentioned in containing nitrogen in addition 

 to the phosphoric acid ; for this reason they are fre- 

 quently called " ammoniated superphosphates," or dis- 

 solved ammoniated bone. 



Advantages of Soluble Phosphoric Acid. Soluble 

 phosphoric acid, in addition to its direct availability, which 



