2O6 SOILS AND FERTILIZERS 



transportation. The price of phosphoric acid in super- 

 phosphates usually ranges from 5 to 6 cents per pound. 

 The field value, that is the increased yields obtained 

 from the use of superphosphates, may or may not be in 

 accord with the commercial value because so many con- 

 ditions influence crop growth. The phosphoric acid ob- 

 tained from feed stuffs is usually considered worth about 

 a cent a pound less than that from superphosphates. 

 Water-soluble phosphoric acid is generally rated a half 

 cent per pound higher than citrate-soluble phosphoric 

 acid. 



234. Phosphate Slag. In the refining of iron ores by 

 the Bessemer process, the phosphorus in the iron is re- 

 moved 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 variable 

 composition. The process by which the phosphorus of 

 pig iron is removed and converted into basic phosphate 

 slag is known as the Thomas process, and the product 

 is sometimes called Thomas' slag. At the present time 

 but little basic slag is produced in this country that is 

 suitable for fertilizer purposes. In Germany and some 

 other European countries large amounts are produced 

 and used. Phosphate slag is ground to a fine powder 

 and is applied directly to the land, without undergoing 

 the sulphuric acid treatment. The phosphoric acid is * 

 present mainly in the form of tetracalcium phosphate 

 (CaO) 4 P 2 5 . 



