90 Fertilizers 



actions in mineral superphosphates, the total available 

 only is regarded, the content of insoluble being 

 ignored. 



CHEMICAL COMPOSITION OF SUPERPHOSPHATES 



As already stated, the composition of the superphos- 

 phates varies according to the richness in phosphoric acid 

 of the phosphates used, and according to the character of 

 the material. Bone-ash and bone-black superphosphates 

 are more uniform in composition than those derived from 

 the mineral phosphates, and the phosphoric acid is practi- 

 cally all in the soluble form. They contain on the average 

 about 16 per cent of total available phosphoric acid. The 

 mineral or rock superphosphates differ from these in being 

 more variable in their total content of available, and in 

 showing wider variations in the proportions of reverted, 

 the latter depending upon the skill in manufacture, as 

 well as the character of the original material. Well-made 

 South Carolina rock superphosphates contain from 12 to 

 14 per cent of total available, of which 1 to 3 per cent is 

 dicalcic, or reverted. There are several grades of the 

 Florida rock superphosphates, due to the variation in the 

 composition of the various raw phosphates. The pebble 

 superphosphates are the richest, often containing as high 

 as 16 or 17 per cent of total available, with varying 

 percentages of reverted and insoluble. The Tennessee 

 superphosphates also vary from the same cause, the richest 

 showing as high as 16 to 18 per cent of total available. 

 The concentrated, or double, superphosphates may contain 

 as high as 45 per cent of available, practically all of which 

 is soluble. The superphosphates made from animal bone 

 are usually more variable in their composition than those 



