COMMERCIAL FERTILISERS 381 



Rock Phosphates. Other sources of large 

 amounts of phosphoric acid are mineral deposits 

 in South Carolina, Georgia, Florida, Tennessee 

 and Canada. These rock phosphates are mostly 

 the fossil remains and the dung of animals, chiefly 

 fish-eating birds. This rock differs widely in 

 composition. South Carolina rock phosphate, dis- 

 covered in 1868, is most generally used. It con- 

 tains 26 to 28 per cent, of phosphoric acid, most of 

 which is insoluble. This South Carolina rock is 

 ground very fine and sold as "floats." Floats are 

 often used for certain crops, especially on moist 

 soils rich in humus. This raw, cheap, slowly- 

 available mineral phosphate may often be used 

 instead of the high-priced, quickly-available 

 superphosphates, especially on perennial crops like 

 fruit trees and grasses. It is becoming quite a 

 common practice to use floats for the main supply 

 of phosphoric acid and to supplement it with small 

 amounts of a superphosphate. 



Florida rock phosphate, discovered in 1888, is 

 more variable in composition than that found in 

 South Carolina. It contains from 18 to 40 per 

 cent, of phosphoric acid. The phosphoric acid 

 in it seems to be much more slowly available than 

 that in South Carolina rock. Tennessee rock 

 phosphate, discovered in 1894, contains 30 to 32 

 per cent, of insoluble phosphoric acid and is used 

 chiefly in making superphosphates. 



Phosphate Slag, also sold as " Thomas slag," 

 "Thomas phosphate meal" and "basic slag" is 

 a by-product in the manufacture of Bessemer 

 steel, phosphorus being an impurity in iron ore. 

 The slag is ground to a fine powder and contains 

 15 to 20 per cent, of phosphoric acid, much of 

 which is soluble in soil water and so is quickly 



