MANURES. 301 



Low class supers contain less than 25 % soluble phosphate 

 expressed as tricalcic phosphate. Medium have between 

 25 and 30 % an I high class have from 30 % upwards. 

 The highest class of supers containing up to 45 % are 

 usually prepared from bone ash and acid. 



All supers contain some insoluble phosphate. This is 

 chiefly Ca 3 (P0 4 ) 2 and runs from 1 to 4 %. As a general 

 rule the amount of this ingredient is not taken into account 

 in valuing the manure. 



Reversion in supers. Supers after standing for some 

 time usually show a fall-eft* in the percentage of soluble 

 phosphate they contain. They are said to have "reverted" 

 to some extent or gone back to the insoluble state. This 

 reversion may be the result of cne or other or both of the 

 following reactions taking place in the mass. 



Either the monocalcic phosphate has interacted with the 

 small quantity of tricalcic phosphate present forming 

 insoluble dicalcic phosphate Ca 2 H 2 (P0 4 ) 2 or it has been 

 acted upon by the sulphates of iron and aluminium 

 present giving rise to the insoluble phosphates of these 

 metals FeP0 4 and A1P0 4 . Reversion from the second 

 cause is more objectionable than that from the first as 

 the phosphates formed are of a more insoluble nature. 

 The insoluble phosphate resulting from reversion is how- 

 ever of more value than original insoluble phosphate. In 

 the first place it is in a very much finer state of division 

 and is more easily rendered available; secondly, dicalcic 

 phosphate is more soluble in soil water than tricalcic is, 



Superphosphate as a manure. When super has been 

 applied to land the first w&tfcr which come* into cfontact 



