182 CHEMICAL FERTILIZERS 



manufacturer will then have to decide whether it is best to 

 attempt to prevent setting or to hasten setting and get it 

 done with. In the former case, he will use acid but dry 

 ingredients ; in the latter case, he will use neutral and 

 damp ingredients, allowing them to stand in store for a 

 period of two to three weeks, and regrinding and passing 

 them through a fine sieve before bagging. 



Sulphate of ammonia tends to dry up the superphosphate, 

 should it be slightly damp, because of the formation of a 

 double compound of calcium sulphate and ammonium 

 sulphate. If in mixing any of the above compounds, a 

 superphosphate of about 30 % be used, then the compound 

 manure produced will vary from 28 % phosphates and i % 

 nitrogen to 20 % phosphates and 7 % nitrogen in the extreme 

 cases quoted on p. 179. As a rule, lower proportions of 

 soluble phosphates are required, hence either a larger amount 

 of insoluble phosphate is used, or a certain amount of gypsum 

 may be employed, to reduce the percentage of soluble phos- 

 phates in the compound. There is a distinct advantage in 

 having two kinds of phosphates, hence a quantity of rock 

 phosphates may be employed for this purpose. Where a 

 superphosphate is mixed with rock phosphate, it is desirable 

 that the rock phosphate selected is one that will give a mini- 

 mum reversion. In Table 21, p. 183, Robertson gives the 

 result of using a Gafsa rock phosphate, mixed with super- 

 phosphate in equal amounts, to measure the degree of 

 reversion. 



According to his experiments, " Gafsa rock phosphate (a 

 North African phosphate) is probably the most suitable for 

 mixing purposes, as it very rarely contains more than 0*75 % 

 of calcium oxide in the form of free carbonate. This phos- 

 phate has the additional but doubtful advantage of being 

 more soluble in 2 % citric acid than any of the other rock 

 phosphates. Egyptian phosphate is probably the next best 

 for mixing purposes, and then come Florida pebble phosphate, 

 and the Makatea Island phosphate, followed by Tunisian 

 and Algerian phosphates. 



" If Gafsa phosphate containing at least 26 % of 



