

406 REPORT OF THE CHEMIST OF THE 
but chemists have, as a matter of convenience, adopted this form 
of statement. So, when the term “phosphoric acid” is used, it 
means simply the compound of phosphorus and oxygen known 
chemically as phosphorus pentoxide or phosphorus anhydride. 
The numbers in the columns under “PHospHoric AciD” state the 
amount of the phosphorus oxide above mentioned, that 1s equivalent 
to the phosphoric acid in the form of phosphates of lime (or calcium) 
actually existing in the fertilizer. 
In the analytical tables, the amount of phosphoric acid is stated 
under three different heads or forms —(Ist) “Available” phos- 
phoric acid, (2d) “ Insoluble” phosphoric acid, and (8d) “Total” 
phosphoric acid. The “phosphoric acid” in these terms is used 
in the sense already explained. 
1. Available phosphoric acid. 
The figures in the column under “available” represent the 
amount of phosphoric acid that is present in the form of soluble 
calcium phosphate, plus the amount present in the form of reverted 
calcuum phosphate. As explained in Bulletin 27, page 420, para- 
graphs 52 and 53, soluble calcium phosphate is formed by treating 
with sulphuric acid some form of insoluble calcium phosphate, 
such as bones, bone-ash, South Carolina rock, etc. The phos- 
phate thus formed is readily soluble in water. Thissoluble phos- 
phate, on standing, is, to some extent, changed into a less soluble 
form, which is still quite readily available as plant food. Since 
both the soluble and reverted forms of phosphates are available as 
plant food, we include in a statement of analysis under the head of 
“ AVAILABLE” PHosPHoric AcID, the phosphoric acid that is present 
in the forms of both soluble and reverted calcium phosphates. 
2. Insoluble phosphoric acid. 
The figures in the column under “ INsoLUBLE” PHosPHORIC AcID 
represent the amount of phosphoric acid that is present in the form 
of insoluble calcium phosphate. This form is not soluble in She 
and is the least valuable form of phosphates. 
3. Total Phosphoric Acid. 
The figures in the column under “ Tota,” PHospHoRio Aor rep- 
resent the entire amount of phosphoric acid present in a fertilizer, 

