MINERALS USED AS FERTILIZERS. 65 



ture of superphosphate. The bone-earth consists in the main 

 of tri-calcic phosphate with from one to two per cent, of cal- 

 cium fluorid (much as in natural apatite), a small amount of 

 magnesic phosphate, and about 4 to 6,% of calcic carbonate. 

 Bone meal can therefore supply to plants both phosphoric acid 

 and nitrogen, and the presence of the latter has been largely 

 the cause of a material overestimate of its efficacy as a fertil- 

 izer in the past. \Yagner's and Maerker's experiments have 

 shown that at least in sandy soils poor in humus, it cannot be 

 considered an adequate source of phosphoric acid for annual 

 crops, and that in these soils its immediate effects are almost 

 wholly due to its nitrogen-content. The slow availability of 

 the phosphoric acid renders it unprofitable as a source of the 

 latter, outside of the heavier lands with abundance of humus ; 

 in "sour" lands (notably on meadows) bone meal produces 

 its best results. In soils naturally calcareous, or in such as 

 have received heavy dressings of lime either as carbonate or in 

 the caustic condition, the manurial effects of bone meal are 

 seriously diminished. Nagaoka ( Bull. Coll. Agr. Tokyo, Vol. 

 6, No. 3) shows that the crop of rice fertilized with bone meal 

 was reduced to less than half when limed, and that the phos- 

 phoric acid taken up by the crop was reduced to one-sixth. In 

 any case it is most important that bone meal should be as finely 

 ground as possible, as in the case of the phosphorites; and this 

 can best be done when it has first been freed from fats by boil- 

 ing with water, and then steamed under pressure. It can then 

 also be most readily converted into superphosphate. 



The phosphate minerals and the fertilizers manufactured 

 therefrom are of primary importance to agriculture. The 

 phosphoric-acid content of soils is mostly very small, and only 

 a fraction of it is usually in an immediately available form. 

 Hence for permanent productiveness, and especially for in- 

 tensive farming or gardening, a cheap supply of phosphate 

 fertilizers is of first importance in all soils and climates. 



Other phosphate minerals occur frequently, but as a rule 

 only in small amounts, in connection with the ores of most 

 metals. The only ones of these of interest to agriculture are 



J'h'ianitc and Dufrcnitc, the phosphates respectively of the 

 protoxid and peroxid of iron. The former occurs in mineral 

 deposits as small blue crystals, or more frequently as blue 

 5 



