l8o SOILS AND FERTILIZERS 



duced by the phosphoric acid but of late there is a ten- 

 dency to restrict the term 'acid phosphate' to the 

 product formed by the action of sulphuric acid, and 

 the term 'super-phosphate 1 to the concentrated product 

 formed by the action of phosphoric acid. 



218. Commercial Value of Phosphoric Acid. — The 

 commercial value of phosphoric acid in fertilizers is 

 determined by the value of the crude phosphate rock, 

 cost of grinding and treating with sulphuric acid, and 

 cost of transportation. The price of phosphoric acid 

 in superphosphates usually ranges from 5 to 6 cents 

 per pound. The field value, that is the increased 

 yields obtained from the use of superphosphates, may 

 not be in accord with the commercial value because so 

 many conditions govern its use. The phosphoric acid 

 obtained from feed-stuffs is usually considered worth 

 about a cent a pound less than that from superphos- 

 phates. Water-soluble phosphoric acid is generally 

 rated a half cent per pound higher than citrate-soluble 

 phosphoric acid. 



219. Phosphate Slag. — In the refining of iron ores 

 by the Bessemer process, the phosphorus in the iron is 

 removed as a basic slag. The lime, which is used as a 

 flux, melts and combines with the phosphorus of the 

 ore, forming phosphate of lime. The slag has a varia- 

 ble composition. The process by which the phos- 

 phorus of pig iron is removed and converted into basic 

 phosphate slag is known as the Thomas process, and 



