128 Report of the Acting Director and Chemist of the 



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 liine. Phosphoric 

 acid stands for a certain amount of phosphate of lime. We may 

 say roughly that one part of phosphoric acid is equivalent to about 

 two parts of phosphate of lime. But we know that phosphoric acid 

 exists in several different forms. (See page 65.) 



(h) Soluhle Phosphoric Acid represents the amount of phosphate 

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

 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 sohible 

 in water. 



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

 phoric acid under certain conditions into which we need not 

 inquire 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 available 

 for the use of plants. 



(f) Soluble and Available Phosphoric Acid is an expression 

 which means the same as available. 



{g) Insolxd)le Phosphoric Acid represents that form of phosphate 

 of lime which is insoluble in water and which is of least value for 

 agricultural purposes. 



(A) 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. 



{%) 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 mis- 

 leading, 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. 



