PHOSPHORUS FERTILIZERS 119 



in this case is a very slow one. The calcium salts of ortho- 

 phosphoric acid are present in all phosphorus fertilizers, 

 and consist of : 



(1) Mono-calcium phosphate or di-acid phosphate of 

 lime, which may be represented by the formula : 



CaH 4 (P0 4 ) 2 or CaO . 2H 2 O . P 2 O 5 



(2) Di-calcium phosphate, or mono-acid phosphate of 

 lime, represented by the formula : 



Ca 2 H 2 (PO 4 ) 2 or 2CaO . H 2 O . P 2 O 5 



(3) Tri-calcium phosphate, or neutral phosphate of lime, 

 represented by the formula : 



Ca 3 P 2 8 or aCaO . P 2 O 6 



In addition to the above, there are ctystalline forms of 

 these substances, viz., the crystalline form of mono-calcium 

 phosphate, represented by : 



CaH 4 (P0 4 ) 2 . H 2 

 that of di-calcium phosphate, with the formula : 



Ca 2 H 2 (P0 4 ) 2 . 4H 2 

 which may also be represented by : 

 CaHPO 4 .*2H 2 O 



Tri-calcium phosphate is a doubtful substance, colloidal 

 and jelly-like in appearance ; no convincing evidence is at 

 hand to settle its exact constitution. 



Another class of phosphorus compounds is represented 

 by the apatite family. True apatite is a fluorine compound, 

 having the formula : 



Ca 5 (P0 4 ) 3 F 



which is a crystalline substance occurring in minerals and 

 in the enamel of animals' teeth. The fluorine may be replaced 

 by chlorine in the above formula, giving chloro-apatite. 

 Most of the mineral sources of phosphates contain such 

 materials. The fluorine in apatite may also be replaced by 

 hydroxyl, giving a hydroxy-apatite, or the fluorine may be 

 replaced by organic matter, giving an organo-apatite. In 

 this last group may be included the chief constituent of bones, 



