CH. VIII j Superphosphate 143 



diiction of a high grade product, finely powdered and 

 dry, free from many of the defects of the older samples. 

 The world's annual production was before the war about 

 10 million tons. 



The rock phosphate comes largely from Northern 

 Africa and it contains other substances besides calcium 

 phosphate : the resulting superphosphate is therefore not 

 entirely constituted as shown in the equation. The rock 

 sometimes contains calcium carbonate, in which case an 

 additional proportion of calcium sulphate is formed. On 

 an average 10 tons of rock phosphate give rise to about 

 18 tons of superphosphate instead of the theoretical 17. 



It has been found convenient to standardise the 

 various grades of superphosphate and sell them on a 

 definite basis. The amount of soluble phosphate is 

 determined by analysis as PoOg and the figure is then 

 calculated as tricalcic phosphate. Thus the ordinary 

 grade contains about 12 per cent. P2O5 soluble in water; 

 this figure is then multiplied by 2-18 to convert it into 

 tricalcic phosphate Ca3(P04)2. Both figures are con- 

 ventional in that superphosphate consists neither of 

 P2O5 nor of Ca3(P04)2, but either figure does very well 

 to express the amount of phosphate soluble in water. 

 The following grades are now obtainable: 



Fixed price 

 f.o.r. 

 " 26 p.c. soluble" equivalent tolI-8 p.c. PgOj 6. 

 "30 p.c. soluble" 13-6 6.10.9. 



"35 p.c. soluble" ., 16-0 7. 10s. 



a sum not exceeding 7s. 6rf. according to time of purchase. 



The student must realise very clearly that the ex- 

 pression "30 per cent, soluble" does not indicate the 

 presence of 30 per cent, of anything in the manure itself. 



*9^ 



