COMPOUND MANURES 



183 



phosphoric acid is used for mixing with superphosphate, the 

 reversion of water-soluble phosphate will be small. But if 

 low grade varieties of rock phosphate, containing 25 % and 

 under of phosphoric acid, are used, the reversion of the water- 

 soluble phosphate to the water-insoluble form in the super- 

 phosphate mixture will be considerable. Superphosphate 

 and rock phosphate produce a very nice mixture, particu- 

 larly if the superphosphate is in a friable condition. 



TABLE 21. 

 GAFSA ROCK PHOSPHATE AND SUPERPHOSPHATE. 



" Mixtures of rock phosphate and superphosphate might 

 be rendered even more efficient if the rock phosphate was 

 ground a great deal finer than it is. As a rule rock phosphate 

 f or^the making of superphosphate is not ground quite so finely 

 as basic slag. From 70 to 80 % only passes a sieve with 

 10,000 holes to the square inch, whereas basic slag has a 

 corresponding fineness of from 80 to 90 %. It is perfectly 

 feasible to have the rock phosphates ground so that upwards 

 of 80 % will pass a sieve with 40,000 holes to the square inch, 

 and there is every reason to suppose that by so doing the 

 efficiency of the mixture with superphosphate would be 

 greatly improved." 



It will be noted that in the quantities which Robertson 

 used the amount of water-soluble phosphoric acid is reduced 



