Superphosphates Potash 83 



dibasic or reverted. One part of the lime in the insoluble 

 is replaced by an equivalent of water, and is expressed as 

 follows : 



Lime 



Lime Phosphoric Acid 



Water 



The reverted form, which means a going back from the 

 soluble toward the insoluble form, is also insoluble in 

 water, but is readily soluble to the roots of plants. 



It was formerly supposed that these three were the only 

 forms in which phosphoric acid existed, but another form, 

 in which four parts of lime are combined with one of phos- 

 phoric acid, and thus called tetrabasic, or tetracalcic, has 

 been found quite recently to exist in the Thomas phosphate 

 powder : 



Lime 



Lime Phosphoric Acid 

 Lime 



This form is insoluble in water, though it has been found 

 to be more available than the insoluble tribasic form. 



HOW SUPERPHOSPHATES ARE MADE 



Any material which contains a high content of the tri- 

 calcic or bone phosphate, 60 per cent or over, is suitable 

 for the manufacture of superphosphates, provided it does 

 not possess a too high content of deleterious substances. 

 In the manufacture of superphosphates, the phosphate is 

 first ground to a fine powder, then mixed with sulfuric acid. 

 The acid dissolves the phosphate, and two parts of the 

 lime which are combined with the phosphoric acid in the 

 tricalcic form are first set free, and then combined with the 



