226 PHOSPHATE FERTILIZERS 



phosphoric acid. In order to prevent any possible excess 

 of sulphuric acid it is customary to add somewhat less than 

 the theoretical amount. This results in the incomplete 

 conversion of all the tricalcium phosphate to monocalcium 

 phosphate. On standing, these two compounds unite to 

 form "reverted" or "gone back" phosphates. That is, 

 the water soluble monocalcium phosphate starts to revert 

 or go back to the tricalcium phosphate and forms dicalcium 

 phosphate. Since the latter is soluble in ammonium citrate 

 solution (Section 212), it is sometimes called citrate soluble 

 phosphoric acid. The equation is as follows: 



CaH 4 (P04) 2 + Ca 3 (P0 4 ) 2 = 2Ca 2 H2(P0 4 )2. 



Reverted phosphate is almost as available as the mono- 

 calcium phosphate. 



(6) Availability. — Acid phosphate is a quick acting fertil- 

 izer, readily available, and all things considered is the best 

 form of phosphate to use under ordinary conditions. 



(c) Effect on the Soil. — In the soil acid phosphate 

 reverts very quickly to dicalcium and tricalcium phosphate. 

 Notwithstanding this reversion, which takes place before 

 the plant obtains much of the fertilizer, acid phosphate is 

 more readily available than tricalcium phosphate or rock 

 phosphate for two reasons : In the first place, acid phosphate 

 dissolves in the soil water and permeates the soil, so that 

 when it is precipitated it is thoroughly distributed. This 

 precipitate is much finer than any mechanically ground 

 material and, moreover, is much better mixed with the soil. 

 In the second place, freshly precipitated di- or tricalcium 

 phosphate is much more soluble in water and carbon dioxide 

 than is tricalcium phosphate, which has been formed for a 

 long time, like the rock phosphate. 



Acid phosphate is said to make a soil acid. This surely 

 is not due to the fact that it is an acid salt, for the plant in 

 the long run uses fully as much phosphoric acid as calcium, 

 and usually more, so that the residual effect could not be 

 acid. Moreover, although it may use up bases by reversion 



