PHOSPHORUS FERTILIZERS 143 



cannot be classed as such. The object of the manure manu- 

 facturers is to produce a superphosphate with the correct 

 mechanical properties, which permit of its easy handling and 

 distribution on the field. These properties depend upon the 

 proportion of free phosphoric acid and water present in the 

 finished article. The water content depends upon the amount 

 of water introduced with the sulphuric acid, whilst the amount 

 of free phosphoric acid depends upon the relative proportions 

 of all the interacting substances, and upon the fineness of the 

 solid portion. The ultimate amount of water left in 

 the superphosphate will depend not merely upon the water 

 which is introduced into the mixer, but will also depend 

 inversely upon the amount of water which is evaporated in 

 the subsequent treatment. When the acid is fairly strong, 

 the temperature of the superphosphate will be raised, and if 

 ample ventilation is given, much water will be evaporated. 

 When the rock phosphate contains much carbonate, large 

 volumes of gas are given of and the temperature rises, both 

 of which conditions assist in evaporation of water. 



When mineral superphosphates are carefully prepared, 

 they may be considered as belonging to the normal type, 

 but much of the commercial superphosphate does not fall 

 into this group. In practice, in normal superphosphate, the 

 amount of water should not exceed 12 %. In a properly 

 prepared* superphosphate, di-calcium phosphate should 

 always be present, and the fraction of phosphoric acid, 

 combined with calcium in the form of di-calcium phosphate, 

 should be equal to that present as free phosphoric acid. The 

 remainder of the soluble phosphoric acid, that is the greater 

 part, is present as mono-calcium phosphate. Mono and di- 

 calcium phosphates are mostly present in the hydrated form, 

 and the calcium sulphate should also have its full amount of 

 water of crystallization. These hydrated salts are formed by 

 secondary reactions in the den. 



Superphosphates of defective constitution show a 

 higher percentage of water than 12 %, and contain 

 more than 2 % of free phosphoric acid reckoned as 

 P 2 O 5 ; di-calcium phosphate is either absent, or only 



