VII. 



SPECIAL MANURES 



147 



Manures similar to superphosphate are made by the action of sul- 

 phuric acid upon bone-ash, bones, or guano. These resemble mineral 

 superphosphates so far as the state of existence of their phosphoric acid 

 is concerned, but may, of course, contain nitrogenous constituents. 



The following table gives the amount of the most important fertilis- 

 ing ingredients in various forms of manures of this class, expressed in 

 the usual conventional way : 



A word of explanation as to the commercial methods of expressing 

 analytical results may be here given. It is the custom to indicate by, 

 say, " 24 per cent soluble phosphates" that 24 is the amount of trical- 

 cium phosphate which contains the same quantity of phosphoric acid 

 as is present in the soluble (monocalcium) phosphates contained in 100 

 parts of manure. 



Since Ca 3 P 2 O 8 corresponds to CaH 4 P 2 O 8 



120 + 62 + 128 40 + 4 + 62 + 128 



310 234 



the amount of real calcium tetrahydrogen phosphate corresponding to 



" 24 per cent soluble " would be 24 x |^J = 18*1 ; but even this is 



not what is meant, for by " monocalcium phosphate " in the trade the 



substance GaP 2 O 6 (really calcium meta-phosphate, which is not 



present in manures) is understood ; so that as this contains the same 



amount of P 2 5 as Ca 3 P 2 O 8 the connection becomes 



CaP 2 6 corresponds; to Ca 3 P 2 O 8 



40 +62 + 96 120 + 62 + 128 



198 310 



Consequently, in the trade, the quantity of monocalcium phosphate 

 equivalent to "24 per cent soluble phosphates" or to "24 per cent 

 bone phosphate rendered soluble, ">as it is sometimes more explicitly 

 described, is given by 



24 x If* = 15-4 per cent. 



By the term "insoluble phosphates" in the above analyses is 

 meant the amount of tricalcium phosphate present as such, together 

 with that equivalent to the dicalcium phosphate or " reverted " phos- 

 phate. The latter is much more valuable than the former and in recent 

 analyses the quantities of each present are given (see Chap. VIII). 



