SUPERPHOSPHATES AND EEEECT OE PHOSPHORIC ACID 83 



calcic phosphate of lime. HalF says it exists as double silicate 

 and phosphate of lime (CaO)., PoOBSiO^. Whatever may be 

 the form of combination of the phosphoric acid in basic slag, it 

 is easily attacked by soil water, and is more available than any 

 of the forms of tricalcium phosphate, though usually less than 

 superphosphate/^ 



Value of Reverted Phosphoric Acid. — The value of reverted 

 phosphate is a subject which has given rise to much dispute among 

 chemists. That it has a higher value than the ordinary insoluble 

 phosphate is now admitted, but in this country, (England) in 

 the manure trade, this is not as yet recognized. At first it was 

 thought that it was impossible to estimate its quantity by chemical 

 analysis. This difficulty, however, has been overcome, and it is 

 generally admitted that the ammonium citrate process furnishes 

 an accurate means of determining its amount. Both on the con- 

 tinent and in the United States reverted phosphoric acid is rec- 

 ognized as possessing a monetary value in excess of that possessed 

 by the ordinary insoluble phosphates. The result is. that raw 

 mineral phosphates containing iron and alumina to any apprec- 

 iable extent are not used in this country (England), although 

 they do find a limited application in America and on the conti- 

 nent.'^ * 



Difference Between Phosphates and Superphosphates. — It is cus- 

 tomary among some farmers to call every fertilizer a phosphate 

 and among others this name is used for the product — superphos- 

 phate. A phosphate is a product containing phosphoric acid as its 

 main ingredient, in the insoluble form, as bone phosphates, rock- 

 phosphates and basic slag phosphates. A superphosphate is a fertil- 

 izer containing principally soluble phosphoric acid. The phosphates, 

 except basic slag and Rodunda phos])hate, may be manufactured 

 into superphosphates by the addition of sulphuric acid as previous- 

 ly mentioned in this chapter. Thus we have superphosphates from 

 bones and minerals, as raw bone superphosphate, steamed bone 

 superphosphate, bone-ash superphosphate, bone-black superphos- 

 phate, Florida hard rock superphosphate, Florida pebble superphos- 



' Fertilizers and Manures. 



s Akiman, Manures and Manuring. 



