SOURCES AND COMPOSITION OF FERTILIZERS 307 



forms water-soluble, reverted, and insoluble. Free phosphoric 

 acid may also be present. The insoluble is the phosphoric acid 

 insoluble in water and in neutral ammonium citrate. It is either 

 the original tri-calcium phosphate of the untreated rock, or phos- 

 phoric acid which as reverted to the insoluble condition. 



When the rock is treated with an excess of acid, some free 

 phosphoric acid or sulphuric acid is present, which rots the bags 

 and also causes the acid phosphate to be very sticky, especially in 

 moist climates, so that it cannot be easily drilled in. 



Available phosphoric acid is the sum of the reverted and water- 

 soluble. In speaking of an acid phosphate, the phosphoric acid 

 referred to is the available. Thus, if we speak of a 16 per cent, 

 acid phosphate, we mean that it is guaranteed to contain 16 per 

 cent, of available phosphoric acid, regardless of the total quantity 

 present. Acid phosphate contains 12, 14, or 16 per cent, available 

 phosphoric acid. The 12 and 14 per cent, grades are often made 

 by mixing dirt or sand with the 16 per cent, phosphate. Such a 

 mixture is not an acid phosphate, but is a mixture of acid phos- 

 phate and dirt or sand. 



Treated phosphates may be made from phosphate rock, apatite, 

 bones, bone ash, or bone black. Whatever the original material 

 used, equal quantities of water-soluble or reverted phosphoric 

 acid have equal values. That is to say, the water-soluble acid 

 produced from apatite is equal in value, pound for pound, to that 

 from bones. 



Dissolved bone should be a treated phosphate prepared from 

 bone. It should contain 2 to 3 per cent, nitrogen. Dissolved 

 bone black is prepared from bone black. 



Superphosphates. Concentrated phosphates are prepared for 

 long distance shipments, when the saving of transportation will 

 more than pay the extra expense of manufacture. High grade 

 phosphate rock is treated with dilute sulphuric acid, with the pro- 

 duction of calcium sulphate and phosphoric acid : 



Ca 3 (P0 4 ) 2 + 3H 2 S0 4 = 3CaS0 4 + 2 H 3 PO 4 . 



