COMMERCIAL FBETILIZERS 513 



phoric acid, the other constituents being calcium carbonate 

 and silica. 



South Carolina phosphate contains from 26 to 28 per 

 cent of phosphoric acid and a very small amount of iron 

 and alumina. As these substances interfere with the 

 manufacture of superphosphate 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 bowlder phosphate. Soft phos- 

 phate contains from 18 to 30 per cent of phosphoric acid, 

 and because of its being more easily ground than most 

 of these rocks it is often applied to the land- without being 

 •first converted into a superphosphate. The other two 

 forms, pebble phosphate and bowlder phosphate, are 

 highly variable in composition, ranging from 20 to 40 per 

 cent in phosphoric acid content. Tennessee phosphate 

 contains from 30 to 35 per cent of phosphoric acid. 



Basic slag, or, as it is also called, phosphate slag or 

 Thomas phosphate, is a by-product in the manufacture 

 of steel from pig-iron rich in phosphorus. The phos- 

 phorus present is usually considered to be in the form of 

 tetracalcium phosphate, (CaO)4P205, or possibly a double 

 silicate and phosphate of lime having the composition 

 (CaO)6P205Si02. It contains also calcium, magnesium, 

 aluminium, iron, manganese silica, and sulfur. Because 

 of the presence of iron and aluminium, and because its 

 phosphorus is more readily soluble than tricalcium phos- 

 phate, the ground slag is applied directly to the soil with- 

 out treatment with acid. 



The degree of fineness to which the slag is ground is 

 supposed to be an important factor in determining its 

 solubility in the soil. It is much more soluble in w^ter 

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