Superphosphates Potash 87 



superphosphates is, however, of greater value than the 

 insoluble in the mineral phosphates, for reasons already 

 given. 



In superphosphates, also, there is nearly always present 

 a greater or less amount depending upon the material 

 of the second form of phosphoric acid, the dicalcic, 

 reverted or retrograde. This form usually exists in the 

 greatest amounts in those made from mineral phosphates, 

 which is believed to be due either to the soluble acting 

 upon the insoluble portions, or to the presence of oxide of 

 iron and alumina, which combine with a portion of the 

 soluble phosphoric acid. The soluble goes back to the 

 less soluble dicalcic form. 



Aikman states the matter very clearly in the following 

 words : 1 "A change which is apt to take place in super- 

 phosphate after its manufacture is what is known as 

 'reversion of the soluble phosphate/ Thus it is found 

 that on keeping superphosphate for a long time the per- 

 centage of soluble phosphate becomes less than it was at 

 first. The rate at which this deterioration of the super- 

 phosphate goes on varies in different samples. In a well- 

 made article, it is practically inappreciable, whereas in 

 some superphosphates, made from unsuitable materials, 

 it may form a considerable percentage. The causes of this 

 reversion are two-fold. For one thing, the presence of 

 undecomposed phosphate of lime may cause it. This 

 source of reversion, however, is very much less important 

 than the other, which is the presence of iron and alumina 

 in the raw material. When a soluble phosphate reverts, 

 what takes place is the conversion of the monocalcic 

 phosphate into the dicalcic. 



"Where reversion is due to the presence of iron and 



1 " Manures and Manuring." 



