MANURES 111 



It is very soluble in water, and is the chief valuable consti- 

 tuent of superphosphates. 



3. As Dicalcium hydrogen phosphate, Qa^Yi^Vfi^, or CaHPO^, 

 a white solid, obtained by precipitating ordinary sodium phos- 

 phate, NajHPO^, with calcium chloride, CaCI, : 



CaClj + Na,HPO, = CallPO, + 2NaCl. 



It is almost insoluble in water, but dissolves in the presence 

 of certain salts — e.g., ammonium citrate. It is probably more 

 easily available for the roots of a plant than tricalcium 

 phosphate. It is found in superphosphates, especially if they 

 have been kept for some time. 



4. As Tricalcium phosphate, CajP^Og. — This is a white sub- 

 stance, almost insoluble in water, but easily soluble in acid. 

 This is the form in which phosphoric acid occurs in bones, 

 mineral phosphates, and most guanos. Its solubility is greatly 

 increased by the presence of carbon dioxide. The solubility, 

 too, depends upon its degree of subdivision and physical state, 

 being much greater if finely divided, amorphous and porous 

 than if coarse, crystalline and compact. 



Most mineral phosphates contain also calcium chloride, 

 CaClg, or calcium fluoride, CaF^. 



5. As Tetracalcium phosphate, Ca^P^Og. This is practically 

 insoluble in water, but dissolves in many saline solutions. It 

 occurs in the slag formed in the dephosphorisation of cast iron 

 by the basic Bessemer or basic Siemens process. 



6. As Ferric phosphate, FePO^, and Aluminium phosphate, 

 AlPO^. These are practically insoluble in water, and nearly 

 so in dilute vegetable acids. Consequently they are difficultly 

 available for plants, and unless extremely finely divided are 

 almost worthless as manures. When they are formed in the 

 soil by the action of ferric or aluminium hydrate upon soluble 

 phosphates, however, they appear to be capable of affording 

 phosphoric acid to the roots of plants, though not readily. 



Generally it may be stated that if a substance is formed by 

 precipitation from solution in the soil itself it will be, as a 



