MINERAL DEPOSITS OF FERTILIZERS 39 



will, as a rule, pass thrjough a standard basic slag sieve 

 containing 100 meshes to the linear inch. In the gravel 

 condition rock phosphates are far too insoluble for manures, 

 but when ground up to the degree of fineness stated above, 

 they can be used directly for this purpose, and have been 

 used very successfully in applications to grazing land or 

 permanent hay. Some records of their having been used 

 successfully for turnips may also be found, but general 

 experience has shown some doubt about the advisability of 

 using rock phosphates for the more intensive forms of 

 cultivation unless accompanied by water soluble phosphate. 

 In practice, rock phosphates are always slow in their action, 

 though in the case of permanent pasture this may be of 

 no great importance. The delay in obtaining a profitable 

 increase from the use of insoluble phosphates may be com- 

 pensated for by the greater economy of the material. For 



