30 PLANT PRODUCTS 



of all the fertilizers. There are a considerable number of 

 slags of low phosphorus content, and it is one of the most 

 important problems before us to utilize these materials. 

 In addition to basic slag there are acid slags produced in the 

 steel industry which do not contain phosphorus. When 

 they possess any value for applying to the soil it is probably 

 due either to their lime content, or to the mere mechanical 

 action of coarse material. 



Mineral Phosphates. — Deposits of mineral phosphates 

 are to be found in many parts of the world ; indeed, on 

 looking at the parts of the world where they have been 

 found one cannot resist the conviction that they have been 

 found just where they have been most looked for, and that 

 probably more extensive search will discover a great many 

 new deposits. The historical ** Cambridge Coprolites " have 

 long since been worked out, and it is chiefly to foreign 

 sources that we now look. Of these the Florida phosphates 

 may be regarded as of the highest quality, containing about 

 75 to 80 per cent, of tri-calcium phosphate. North Africa 

 and the Pacific Islands provide us with some materials of 

 slightlylowergrade, whilst Australasia possesses some valuable 

 deposits. These materials, if finely ground, can be applied 

 directly to the land. They are, however, much less soluble 

 than basic slag, and for direct application to the land it 

 certainly seems a little contradictory for England to export 

 basic slag and to import mineral phosphates. Where 

 mineral phosphates have been systematically applied to 

 pasture, in comparison with basic slag, some quite good 

 results have been obtained. Satisfactory results have also 

 been found when mineral phosphates have been used with 

 the turnip crop. Nearly all the mineral phosphates actually 

 mined are used for the manufacture of super-phosphate. 

 The manufacture of this is described in other volumes of this 

 series, and need only here be briefly alluded to. 



The mineral phosphate, having been finely ground, is 

 treated with sulphuric acid, and is run into a " den," 

 where the reaction is completed. As the resulting material 

 is apt to be sticky, it is sometimes, after breaking up, dusted 



