SOi Bureau of Farmers' Institutes. 



Phosphoric Acid. 



(a) Phosphoric Acid, as used in connection with fertilizers, is a 

 compound containing phosphorus and oxygen, which in fertilizers 

 is found never by itself, but in combination with lime. Phos- 

 phoric acid stands for a certain amount of phosphate of lime. 

 We may say roughly that one part of phosphoric acid is equiva- 

 lent to about two parts of phosphate of lime. But we know that 

 phosphoric acid exists in several different forms. (See page 227.) 



(&) Soluble Phosphoric Acid represents the amount of phosphate 

 of lime that dissolves easily in water. As explained already, sol- 

 uble 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 phosphate thus formed is 

 readily soluble in water. 



(c) Reverted Phosphoric Acid is formed from soluble phos- 

 phoric acid under certain conditions into which we need not in- 

 quire here. Suffice it to say, that the soluble compound of phos- 

 phoric acid often changes, to some extent, on standing, into a 

 form, which, while less soluble, is still quite readily available as 

 plant food. 



(d) Precipitated Phosphoric Acid is simply another name for 

 the reverted form. 



(e) Available Phosphoric Acid includes both the soluble and 

 reverted forms of phosphoric acid, because both forms are avail- 

 able for the use of plants. 



(/) Soluble and Available Phosphoric Acid is an expression 

 which means the same as available. 



(r/) Insoluble Phosphoric Acid represents that form of phos- 

 phate of lime which is insoluble in water and ammonium citrate 

 solution and which is of least value for agricultural purposes. 



(h) Total Phosphoric Acid represents the entire phosphoric acid 

 compounds without regard to the forms in which they exist. The 

 total phosphoric acid is, therefore, the sum of the soluble, reverted 

 and insoluble forms, or, to state it in another way, the sum of 

 the available and insoluble forms. 



( i) Phosphoric Acid equal (or equivalent) to Bone Phosphate 

 of Lime is an expression which usually means nothing more or 

 less than insoluble phosphoric acid. The expression is apt to be 

 misleading, as it appears to imply that the phosphoric acid is 

 derived from bone. It is applied probably to ground rock even 

 more often than to bone. The expression is used often appar- 



