Phosphates Their Sources 73 



with the grade of ore and the amount of lime used in the 

 process of manufacturing steel. The imported product 

 usually contains 15 to 19 per cent of phosphoric acid. 

 Among the chief constituents, other than phosphorus, 

 are a number of compounds containing calcium, mag- 

 nesium, iron, manganese and silicon. In general the 

 composition will range within the limits following: 



Phosphoric acid . . . . 12 to 20 per cent 



Calcium oxide . . . . 35 to 50 per cent 



Magnesium oxide .... 4 to 6 per cent 



Manganese oxido .... 5 to 10 per cent 



Iron oxide 12 to 18 per cent 



Silica 4 to 8 per cent 



Alumina 1 to 3 per cent 



The availability of basic-slag is dependent in large 

 measure upon the fineness of division and soil conditions. 

 From 80 to 90 per cent of the total phosphoric acid con- 

 tained in it is guaranteed available, but experimental 

 evidence shows that the phosphoric acid in basic slag is 

 only a little more than one-half as quickly available as 

 soluble calcium phosphate. The lime contained is un- 

 doubtedly an asset. Good results have been obtained 

 on low wet soils, but it is only when the material is used 

 in large quantity that the effects of the lime become 

 noticeable. 



Manufactured phosphates. 



There are a number of phosphatic materials manu- 

 factured in one way or another from minerals bearing 

 phosphorus which are mentioned more as a matter of 

 interest than of practical importance. 



Artificial basic-slag weal. Many attempts have been 

 made to produce by manufacture a material similar to 



