? TOBACCO CULTURE. 



PHOSPHORIC ACID. 



(a) Phosphoric Acid, as used in connection with fertili- 

 zers, is a compound containing phosphorus and oxygen, 

 which in fertilizers is never found by itself, but in combina- 

 tion with lime. 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 phos- 

 phate of lime. But we know that phosphoric acid exists in 

 several different forms. 



(b) Soluble Phosphoric Acid represents the amount of 

 phosphate of lime that dissolves easily in water; it is formed 

 by treating with-sulphuric acid some form of insoluble lime 

 phosphate, sucli as bones, phosphate rock, etc. The phos- 

 phate thus formed is readily soluble in water. 



(t) Reverted Phosphoric Acid is formed from soluble 

 phosphoric acid under certain conditions into which we 

 need not inquire here. Suffice it to say that the soluble 

 compound of phosphoric acid often changes to some extent, 

 on standing, into a form, which while less soluble, is still 

 quite readily available as plant food. 



((/) Available Phosphoric Acid includes both the sol- 

 uble and reverted forms of phosphoric acid, because both 

 forms are available for the use of plants. 



(e) Soluble and Available Phosphoric Acid is an ex- 

 pression which means the same as available. 



(/) Insoluble Phosphoric Acid represents the form of 

 phosphoric acid in raw phosphate of lime, and which is of 

 least value for agricultural purposes. 



