Sao 



ANNUAL REPORT OF THE 



Off. Doc. 



( 3 ) INSOLUBLE PHOSPHORIC ACID. 



This is known under several names, as "Insoluble calcium phos- 

 phate," "Tri-calcium phosphate," "Bone phosphate of lime" and "Nor- 

 mal calcium phosphate." This form is called insoluble, because it 

 does not dissolve in water or weak acids, as does the soluble or re- 

 verted phosphoric acid, but requires some strong acid to cause its 

 decomposition or solution. 



Insoluble phosphoric acid is found in nature in large quantities, 

 some of the chief sources of which will be noticed later. 



Insoluble phosphates are found everywhere in the soil and most 

 of them are of but little value to the farmer, because thev are not 

 easily dissolved and can, therefore, be utilized but slow^ly by plants. 



(4) TETRA-CALCIUM PHOSPHATES. 



Tetra-calcium phosphate, or four-lime phosphate, is a form of 

 phosphoric acid of recent discovery, and has been found to exist in 

 slag phosphates. It contains more lime in proportion to phos- 

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

 in water, it has been found to be more available to plants than, in- 

 soluble phosphate of lime (tri-calcium phosphate). 



The following table gives the chemical composition and differences 

 of the four phosphates of lime: 



TABLE 5. 

 Chemical Composition of the Four Phospliates of Lime. 



