300 EGYPTIAN AGRICULTURE. 



to the presence of iron and aluminium compounds in the 

 phosphate will be brought forward immediately in dealing 

 with the question of the reversion of the soluble phosphate 

 of supers. The skill of the maufacturer lies in turning 

 out, from the raw phosphate at his disposal, a manure 

 containing as much soluble phosphate as possible and at 

 the same time of a fine earthy texture, easy to handle and 

 apply. More acid may give a little more soluble phosphate 

 but if the manure becomes a sticky paste in consequence 

 the difficulty in dealing with it far outweighs the benefit- 

 derived from the increased solubility. 



Superphosphates are therefore manures containing 

 primarily monocalcic phosphate and gypsum, with some 

 tricalcic phosphate, sandy matters and usually small quan- 

 tities of sulphates and phosphates of iron and aluminium. 



They are divided into low, medium and high -class 

 supers according to the percentage of soluble phosphate 

 they contain. In the purchase of superphosphate, especially 

 when large quantities are in question, a guarantee of 

 composition should be given by the seller. In such a 

 guarantee the percentage of soluble phosphoric acid is 

 usually expressed thus: 



Soluble phosphoric acid P 2 5 = 13'5^, equal to 



Soluble phosphate of Iime-Ca 3 (P0 4 ) 2 = 29'4% 

 although the acid is present in the manure as monocalcic 

 and not as tricalcic phosphate. This really means that in 

 this particular sample the amount of soluble monocalcic 

 phosphate CaH 4 (P0 4 ) 2 is equal to what would be for- 

 med by the conversion of 29 '4 % of insoluble tricalcic 

 phosphate into the soluble form. 



