SUPERPHOSPHATES AND EFFECT OF PHOSPHORIC ACID 87 



and generally three times as much as in superphosphates. Wiley^ 

 suggests that superphosphate is a more correct name for this , 

 class of material as it is a phosphate acted upon by free phos- 

 phoric acid and superior to acid phosphate. Phosphates contain- 

 ing too low a percentage of phosphate of lime for profitable manu- 

 facture of acid phosphate may be utilized in obtaining the free 

 phosphoric acid. 



Not much double superphosphate is found on the American 

 market but it is quite popular in Germany where it is manufac- 

 tured principally. Double superphosphates contain about 40 to 

 45 per cent, of available phosphoric acid. They contain less im- 

 purities than acid phosphates. The phosphoric acid is present 

 in the same forms as in acid phosphate, namely as soluble, re- 

 verted and insoluble phosphoric acid. Double superphosphates 

 are expensive but sometimes economical to purchase when freight 

 is high. 



No Free Acid in Treated Phosphates. — Acid phosphates and 

 double superphosphates when well manufactured do not contain 

 any free acid as all of the sulphuric acid is united with lime and 

 forms gypsum. Of course it is possible for a manufacturer to 

 make a product that will contain free acid, but this is not done 

 and the product delivered to the trade does not contain any free 

 acid. 



The Color of an Acid Phosphate. — There seems to be a prefer- 

 ence among some for a light colored acid phosphate while others 

 demand a dark colored product. The color and nature of the 

 raw material from which acid phosphates are made determine 

 their final color. The manufacturers in order to satisfy the trade 

 often carry two different colored acid phosphates of the same 

 chemical composition which are made from the same raw product. 

 The dark or black color is obtained by mixing in lamp-black 

 when the final product is not sufficiently dark. Some raw ma- 

 terials as bone, bone-black, etc., produce a black superphosphate 

 without the addition of any coloring substance. The color of 

 an acid phosphate does not indicate its fertilizing value.* 



' Principles and Practice of Agricultural Analysis, Vol. II. 

 7 



