COMMERCIAL FERTILIZER MATERIALS 455 



immediate source of phosphorus. In the soil, water charged 

 with carbon dioxide slowly converts the insoluble tricalcium 

 phosphate into the soluble mono-calcium form: 

 Ca 3 (POJ 2 + 4C0 2 + 4H 2 = CaH 4 (P0J 2 + 2CaH 2 (C0 3 ) 2 



255. Rock phosphate 1 (Ca 3 (Po 4 ) 2 -}-). — There are many 

 natural deposits of mineral phosphates in different parts of 

 the world, some of the most important of which are in North 

 America. The phosphorus in all of these is in the form of 

 tricalcium phosphate, but the materials associated with it 

 vary greatly. Rock phosphate may occur in nature as soft 

 phosphate, pebble phosphate, boulder phosphate, and hard 

 rock phosphate. 



South Carolina phosphate contains from 26 to 28 per cent, 

 of phosphoric acid and a very small amount of iron and 

 aluminum. As these latter substances interfere with the man- 

 ufacture of acid phosphate from rock, their presence is very 

 undesirable, rock containing more than from 3 to 6 per cent, 

 being unsuitable for that purpose. 



Florida phosphates exist in the form of soft phosphate, 

 pebble phosphate, and boulder phosphate. Such phosphate 

 contains from 18 to 30 per cent, of phosphoric acid, and be- 

 cause of its being softer than most of these rocks it is often 

 applied to the land without being first converted into a soluble 

 form. The other two forms, pebble phosphate and boulder 

 phosphate, are highly variable in composition, ranging from 

 20 to 40 per cent, in phosphoric acid content. Tennessee 

 phosphate, which is now very important, contains from 25 to 

 35 per cent, of phosphoric acid. 



Rock phosphate, or floats as it is often called, appears on 



the market as a heavy finely ground powder of light gray 



color. It generally carried about 27 per cent, of phosphoric 



acid as Ca 3 (P0 4 ) 2 . A typical analysis is as follows: 



1 Waggaman, W. H v and Fry, W. H., Phosphate Rock and Methods 

 Proposed for Its Utilisation as a Fertiliser; U. S. Dept. Agr., Bui. 312, 

 1915. 



