234 Bureau of Farmers' Institutes. 



bone-ash, etc., with sulphuric acid. With high grade material, 

 equal parts of sulphuric acid and rock-phosphate axe used in 

 dissolving the insoluble phosphate of lime. By this treatment 

 there is formed a mixture of soluble phosphate of lime and 

 gypsum (sulphate of lime) in which there are about one and 

 one-half pounds of gypsum for each pound of soluble phosphate. 

 Owing to their comparative cheapness and abundance, rock- 

 phosphates are more often used in making superphosphates than 

 bones, bone-ash or bone-black. The value of a superphosphate 

 depends upon the amount of soluble phosphate of lime present 

 in it together with the amount of reverted phosphate of lime. 

 The amount of soluble phosphoric acid compounds in superphos- 

 phates varies with the kind of phosphate used in making super- 

 phosphates, and also with other conditions which Ave need not 

 mention here. Good quality of dissolved bone contains 12 to 

 18 per cent, of soluble phosphoric acid. Dissolved bone-black 

 contains from below 15 to 17 per cent, of soluble phosphoric acid. 

 Superphosphate made from rock-phosphate may contain from 12 

 to 18 per cent, of soluble phosphoric acid. 



' Double superphosphate " is a name applied to a concentrated 

 form of soluble phosphoric acid. It is chiefly manufactured in 

 Europe and is little used in the United States. Double super- 

 phosphate contains 35 to 45 per cent, of phosphoric acid in soluble 

 forms. 



Thomas Slag is more familiarly known as odorless phosphate. 

 It is also known under several other names, such as basic iron 

 slag, Thomas scoria, phosphate slag, etc. This is a comparatively 

 new source of phosphoric acid compounds. It is a by-product 

 formed in the manufacture of iron and steel from certain kinds 

 of iron ore — containing phosphorous compounds. In the pro- 

 cess a phosphate of lime is formed, which is ground to a fine 

 powder. Odorless phosphate is insoluble in water, but is some- 

 what soluble in ammonium citrate solutions and its phosphoric 

 acid is, therefore, available to some extent. The phosphoric acid 

 is in the form known as tetra-calcium phosphate. It contains 

 15 to 20 per cent, of phosphoric acid, about one-third of which is 

 citrate-soluble, and, in addition, large proportions of lime and 

 iron oxide. The practical results coming from the use of this 

 form of phosphoric acid compounds have been varied, often being 

 most excellent and again very indifferent. 



In the table following, we give the amounts of different forms 

 of phosphoric acid found in different phosphate materials: 



