ROCK PHOSPHATE, FLOATS 223 



172. Dissolved Bone-Black. — If the spent bone-black 

 above mentioned is treated with sulphuric acid, it becomes a 

 readily available phosphate fertilizer, being converted into 

 monocalcium phosphate which is. soluble in water, whereas 

 tricalcium phosphate is only slowly soluble in water and 

 carbon dioxide. The reaction may be expressed thus: 



Cas(P04)2+2H2S04=CaH4(P04)2+2CaS04 



Its content of phosphoric acid is about 14 to 16 per cent. 



173. Rock Phosphate, Floats. — Originally the name "floats" 

 was applied to a particularly fine ground rock phos- 

 phate, so fine that it would float in the air. Now, however, 

 the term is loosely used for any finely ground rock phosphate. 

 Deposits of phosphate rock are found in many places. In 

 France, Belgium, Portugal, and North Africa there are 

 beds of greater or less thickness. The greatest source of 

 supply, however, comes from the United States, in Florida, 

 South Carolina, and Tennessee. Recently there have been 

 discovered immense beds in Idaho, Wyoming, and Montana. 

 These constitute a reserve supply of great importance since 

 the older mines are being rapidly exhausted. In South 

 Carolina and Florida the phosphate occurs largely as 

 pebbles or boulders in deposits resembling gravel beds 

 (Fig. 52). In Tennessee it occurs in veins or pockets. 



The phosphorus occurs as tricalcium phosphate together 

 with varying amounts of iron and aluminium compounds 

 and calcium carbonate. The phosphoric acid content 

 varies from 25 to 40 per cent., iron and aluminium oxide, 

 2 to 6 per cent., and calcium carbonate from 1 or 2 per cent, 

 to 10 or 15 per cent. 



As a fertilizer, rock phosphate is very slow acting when 

 applied alone. The finer it is, the more valuable it becomes, 

 but even very finely ground it is best used in connection 

 with decaying organic matter. Applied to acid peat or muck 

 soils it is especially good, and when mixed with manure 

 gives excellent results. Opinion is divided as to its value 

 compared to acid phosphate, even when mixed with manure. 

 Whether or not it is as good as, or better than, acid phosphate 



