112 ELEMENTARY AGRICULTURAL CHEMISTRY 



rule, much more available for plants than if it were applied 

 ready-formed to the soil, even in a finely divided condition. 



The chief commercial forms in which phosphates are used as 

 manures are : 



1. Mineral phosphates, consisting largely of tricalcium 

 phosphate, Ca3P208. Immense quantities of this material are 

 found in the United States of America, in Belgium, Algeria, 

 and Canada. Generally they are not used directly, but are 

 converted into superphosphate. If very finely ground, how- 

 ever, they are occasionally successful. 



2. Mineral Superphosphates. — These are made by treating 

 the raw phosphates with sulphuric acid (chamber acid of sp. gr. 

 1*55), which produces the reaction already described. A super- 

 phosphate contains a considerable portion of its phosphoric 

 acid in the form of monocalcium tetrahj drogen phosphate, 

 CaH4P208, some as unchanged Ca3P208, and generally some as 

 CaHP04. 



3. Dissolved jBones.-^ -These are made by a process similar 

 to the last described, but the product contains nitrogenous 

 matter, and usually a larger propoi tion of unchanged tricalcium 

 phosphate. 



4. Basic Slag, Thomas Slag, or Thomas Phosphate. — This 

 product is formed in the basic Bessemer process of making 

 steel from cast iron. It contains its phosphorus pen^oxide 

 (usually from 16 to 18 per cent.) in the form of tetracalcium 

 phosphate, Ca3P208, together with free lime, and is particularly 

 suitable for soils rich in organic matter. It is only efiective if 

 applied in a finely divided state, so that its value largely 

 depends upon the thoroughness of the grinding ; 80 to 90 per 

 cent, should pass a sieve with 100 meshes to the linear or 

 10,000 meshes to the square inch. Its phosphoric acid, though 

 insoluble in water, is readily soluble in saline solutions — e.g., 

 solution of ammonium citrate. The composition of the com- 

 mercial product varies considerably, but a good sample contains 

 about 18 per cent, of phosphorus pentoxide, 45 per cent, of 

 lime, 15 per cent, of ferric oxide, and small quantities of silica, 

 magnesia, alumina, &c. 



