The Composition and Use of Fertilizers. 229 



contained in them. Soluble phosphate of lime, whatever its 

 source, whether bone or rock, is one and the same chemical com- 

 pound, and as such has the same value as plant food. 



(3) Reverted 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 some 

 time, it will happen under certain conditions that some of the solu- 

 ble 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 contain- 

 ing carbonic acid or salts of ammonia. Since the soil and plant 

 roots generally contain acids sufficiently 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 term " 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 considerable extent in 

 bones and other forms of organic matter. 



(4) Tetra-calcium Phosphate. 



Tetra-calcium phosphate is a form of phosphate of lime found 

 in phosphate slags. It contains more lime in proportion to phos- 

 phoric acid than any other form of phosphate. While it is in- 

 soluble in water, it has been found to be more available as plant 

 food than insoluble phosphate of lime. 



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

 as food for plants, we have: 



1. The ordinary insoluble phosphate of lime, which can be 

 changed by treatment with sulphuric acid into — 



2. The soluble phosphate of lime, and this, on standing, may, 

 under certain conditions, undergo change, forming — 



3. 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. 



