228 Bureau of Farmers' Institutes. 



(3) Reverted phosphate of lime, or di-calcium phosphate. 



(4) Tetra-calciimi phosphate. 



(1) Insoluble Phosphate of Lime. 



This is known under several other names, as " insoluble cal- 

 cium phosphate," " normal calcium phosphate," " tri-calcium phos- 

 phate," " bone phosphate of lime," etc. 



This form of calcium phosphate is called insoluble because it 

 does not dissolve in water. 



It is found in nature in large quantities in several minerals, 

 which will be noticed later. It also constitutes about 85 per cent, 

 of the ash or inorganic matter of bones. It is also contained in 

 the excrement of animals, as in guano, etc. 



Insoluble phosphate of lime is found everywhere in the soil. 

 However, in this form, calcium phosphate has the least value for 

 the farmer, because it is not easily dissolved and cannot, there- 

 fore, be taken up and used by plants, except very slowly. To 

 make the insoluble phosphate available for plants so that they can 

 take it up, the insoluble phosphate must be converted into some 

 form which is soluble; that is, which dissolves in water. This 

 can be done by treating it with sulphuric acid (oil of vitriol). 



(2) Soluble Phosphate of Lime. 



This is known under several other names, as " acid phosphate 

 oi lime," " acid calcium phosphate," " acid phosphate," " super- 

 phosphate of lime," " superphosphate," " mono-calcium phos- 

 phate," etc. It is not found naturally occurring. 



As indicated above, the soluble calcium phosphate is made by 

 treating insoluble calcium phosphate with sulphuric acid. By 

 this treatment, a portion of the calcium is removed from the phos- 

 phate and unites with the sulphuric acid, forming sulphate of 

 lime or gypsum in addition to the soluble phosphate. This mix- 

 ture of the soluble phosphate and gypsum is known as " super- 

 phosphate of lime." The phosphate in this form, being easily 

 soluble in water, can be readily taken up by plants, and is, there- 

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

 known to have value as an indirect fertilizer. In plain superphos- 

 phate of lime there are generally formed about 116 pounds of 

 water-free sulphate of lime (equivalent to 147 pounds of gypsum) 

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

 superphosphates depends upon the amount of soluble phosphate 



