SECTION III.-PHOSPHOEUS FERTILIZERS 



Phosphorus Compounds in Fertilizers. The phosphorus 

 compounds found in fertilizers do not exhibit the same 

 variety of form as do the nitrogen compounds in the nitrogen 

 fertilizers. Whereas nitrogen occurs in combination with 

 hydrogen as ammonia, or with oxygen as nitric acid, or with 

 carbon as cyanamide and organic nitrogen compounds, the 

 phosphorus fertilizers are all substances containing some 

 form of calcium phosphate. The varieties of calcium phos- 

 phate are, however, as numerous as the different forms of 

 nitrogen in the nitrogen fertilizers. 



Of the oxides of phosphorus only the pentoxide is present 

 in fertilizers, and of the acids formed from phosphorus it is 

 only pentavalent phosphorus that need be considered. The 

 theoretical pentahydrate of phosphorus does not appear, 

 but dehydrated forms thereof constitute the whole of the 

 phosphorus compounds. Ortho-phosphoric acid, which may 

 be represented as 



H 3 P0 4 or PO(OH) 3 or |(P 2 O 5 . sH 2 O) 



is the most commonly occurring form in which phosphorus 

 occurs. Pyrophosphoric acid, which may be represented by 



H 4 P 2 O 7 or P 2 O 3 (OH) 4 or P 2 O 5 . 2H 2 O 



only occurs in substances which have been subjected to heat. 

 Metaphosphoric acid, which may be represented as 



HPO 3 or PO 2 . OH or J(P 2 O 5 . H 2 O) 



is a product of overheating. The two latter forms, pyro and 

 metaphosphoric acid, on boiling in acid solutions, are both 

 converted into ortho-phosphoric acid. A similar change 

 takes place in the presence of much water, even when the 

 solutions are cold and neutral or alkaline, but the reaction 



