66 Eeport of the Acting Director and Chemist op the 



therefore, of great value as a fertilizer. The sulphate of lime is also 

 known to have value as a fertilizer. In plain superphosphate of 

 lime, there are generally formed about 116 pounds of sulphate of 

 lime for each 100 pomids of soluble phosphate of lime. The value 

 of superphosphates depends upon the amount of soluble phosphate 

 contained in them, 



(3) Beverted Phosphate of Lime. 



Reverted phosphate of lime is known also as " reverted calcium 

 phosphate," "precipitated phosphate of lime," "di-calcium phos- 

 phate," " citrate-soluble phosphate," etc. 



When soluble phosphate of lime is allowed to stand for sometime, 

 it will happen under certain conditions that some of the soluble 

 phosphate is changed into a less soluble form of phosphate. This is 

 not the same form as ordinary insoluble calcium phosphate above 

 described ; for a reverted phosphate, while insoluble in water, can 

 be readily dissolved by weak acids or by water containing carbonic 

 acid or salts of ammonia. Since the soil and plant roots generally 

 contain acids suihciently strong to dissolve reverted phosphates, 

 phosphoric acid in this form is generally regarded as very nearly 

 equal to soluble phosphates in value as a fertilizer. The tenn 

 "reverted" was introduced to express the fact that the phosphoric 

 acid in this form had once been soluble in water but that it had 

 "reverted" or "gone back " to a form insoluble in water. 



The reverted form of phosphoric acid is often found in small 

 quantities in connection with insoluble phosphates, and in larger 

 amounts, in guanos; it is also found to a considei^able extent in 

 bones and other forms of organic matter. 



Summary. — Of the forms of phosphate of lime which are used as 

 food for plants, we have 



First, the ordinary insoluble phosphate of lime, which can be 

 changed by treatment with sulphuric acid into 



Second, the soluble phosphate of lime, and this, on standing, may, 

 under certain conditions, undergo change, forming 



Third, the reverted phosphate of lime, which is insoluble in pure 

 water but soluble in the acids of the soil and plants and in water 

 containing carbon dioxide. 



The soluble and reverted forms of phosphoric acid, taken together, 

 are called available phosphoric acid. 



