PHOSPHATIC MANURES 219 



acid which remains, until it is practically all converted 

 into monocalcic phosphate according to the following 

 equation : 



Ca 3 (P0 4 ) 2 + 4H 3 P0 4 = 3CaH 4 (P0 4 ) 2 



All that is accomplished by the process, therefore, is the 

 elimination of the calcium sulphate, and under ordinary 

 circumstances, the advantages of this extreme concentra- 

 tion are not apparent. 



Three and a-half tons of 25 per cent, quality contain 

 exactly the same amount of phosphates as two and a- 

 half tons of 35 per cent, quality, and have precisely the 

 same fertilising power. If they can be obtained at the 

 same rate per unit of phosphate, the latter would be 

 the more economical to the extent of the saving in the 

 cost of transport and handling. Apart from this, there 

 is no difference between the two. The price is generally 

 proportional to the percentage of soluble phosphate, but 

 standard qualities can, as a rule, be obtained at lower 

 rates than those which have to be specially prepared. 



Reverted Phosphates. When superphosphates are kept 

 for some time, the soluble phosphate gradually changes 

 back into an insoluble form called reverted or reduced 

 phosphate. Eeversion has been attributed to the forma- 

 tion of dicalcic phosphate by the interaction of the mono- 

 calcic and tricalcic compounds. Superphosphates do not, 

 however, contain enough undissolved tricalcic phosphate 

 to account for the extent to which the change has been 

 observed to take place, and also on other grounds, the 

 explanation is improbable. It is possible, however, that 

 reversion may be due, in some cases, to the production 

 of dicalcic phosphate from monocalcic phosphate itself 

 according to the equation : 



CaH 4 (P0 4 ) 2 = CaH 4 P0 4 + H 3 P0 4 



Monocalcic Dicalcic Phosphoric 



phosphate, phosphate. acid, 



