306 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Thomas phosphate is a by-product from the manufacture of 

 steel from phosphatic pig iron. It contains 15 to 20 per cent, 

 phosphoric acid in connection with large amounts of lime and 

 oxide of iron. The phosphoric acid was believed to be present 

 as tetra-calcium phosphate, but according to Morison, 1 it is a silica 

 phosphate of lime and ferrous iron. Thomas slag has its greatest 

 effect upon soils rich in organic matter and poor in lime. It con- 

 tains free lime, which may neutralize soil acids. It is a slow 

 acting fertilizer. 



Acid Phosphate. It has been found by experiment that treat- 

 ment, of phosphates with sulphuric acid exerts a powerful in- 

 fluence upon their crop-producing power, and immense quantities 

 are so treated for this reason. The rock is first ground to a 

 powder, and treated with approximately an equal weight of sul- 

 phuric acid. The following reaction takes place : 



(1) Ca 3 P 2 O 8 + H 2 SO 4 = H 3 PO 4 + CaSO 4 . 



(2) Ca 3 P 2 8 + 4H 3 P0 4 == 3CaH 4 P 2 O 8 . 



Mono-calcium phosphate soluble in water is produced from cal- 

 cium phosphate. The calcium sulphate, or gypsum, unites with 

 water and causes the mass to harden. On standing the following 

 reaction may take place, di-calcium phosphate being formed : 



(3) CaH 4 P 2 O a + Ca,P 2 0, = 2Ca 2 H 2 P 2 O 8 . 



This process is called reversion, and the di-calcium phosphate 

 is termed reverted phosphoric acid. Reversion is also caused by 

 the presence of iron and aluminium. The reaction is not clearly 

 understood but may possibly be as follows : 



(4) 2CaH 4 P 2 8 + Fe 2 3 = Fe, 2 P 2 O 8 + Ca,H 2 P 2 O 8 + 3H 2 O. 

 The reverted phosphoric acid is assumed to have a value equal 



to water-soluble phosphoric acid. It is also termed citrate-soluble 

 phosphoric acid, since it is dissolved by ammonium citrate in the 

 chemical analysis of the fertilizer. Reversion by iron oxide and 

 alumina produces ferric or alumina phosphate, both of which 

 contain the phosphoric acid in an insoluble form. Some alumina 

 phosphates are, however, citrate-soluble. 



Phosphoric acid is thus present in an acid phosphate in three 

 1 Jour. Agr. Sci., 1909, p. 161. 



