SUPERPHOSPHATES AND EFFECT OF PHOSPHORIC ACID 85 



phoric acid. The insoluble phosphoric acid is not soluble in this 

 citrate solution but it is soluble in strong acids, hence the names 

 citrate insoluble and acid soluble are applied to insoluble phos- 

 phoric acid. 

 Reverted phosphoric acid is equivalent to citrate soluble phosphoric acid. 



T»,o^i„Ki^ r.v,^o.,T,^^;^ ^^iA ;.^^„j oi^„f f^ ' citrate insoluble phosphoric acid 

 Insoluble phosphoric acid IS equivalent to s •, , ,, 1 *^i. ■ j 



^ ^ ^ ( acid soluble phosphoric acid. 



The sum of the soluble and reverted phosphoric acid is called 

 available phosphoric acid, or the sum of the soluble and citrate 

 soluble phosphoric acid is available phosphoric acid. The farmer 

 often confuses the term acid soluble as belonging to the available 

 phosphoric acid on account of the use of the word soluble. 

 Again, the difference between the total phosphoric acid (which is 

 the sum of the soluble, reverted and insoluble forms) and the 

 insoluble phosphoric acid is available phosphoric acid.* 



The Difference of the Forms of Phosphoric Acid in Superphos- 

 phates. — In the manufacture of superphosphates not all of the 

 tricalcium phosphate is converted into soluble phosphoric acid. 

 The manufacturer generally calculates to add just enough acid 

 to convert most of the phosphoric acid into the soluble form. 

 However, he does not wish to add too much acid in order to put 

 out a profitable marketable product. Hence most of the superphos- 

 phates found on the market contain some insoluble phosphoric 

 acid, ranging perhaps from a few hundredths to as high as four 

 per cent, in poor acidulation. This insoluble phosphoric acid 

 in superphosphates is different. That in the lx)ne superphos- 

 phates is of more value as regards availability than the in- 

 soluble phosphoric acid in the mineral superphosphates. The in- 

 soluble phosphoric acid is also of different value in the mineral 

 superphosphates depending upon the nature or purity of the rock 

 from which they were made. However, the insoluble phosphoric 

 acid in super or acid phosphates is generally present in small 

 amounts and would only have to be seriously considered when the 

 acidulation proves insufficient. The soluble phosphoric acid in all 

 superphosphates is the same, whether the superphosphates are 

 made from lx)nes, bone-ash, bone-black, or any of the mineral 

 phosphates. It is an erroneous opinion among some, that the 



