This form still continues to take on lime and by and by gets back to the original insoluble form called insoluble phosphoric acid.

The soluble phosphoric acid and reverted phosphoric acid are available to plant roots.  The insoluble form is not.

The rock phosphates contain from 26 to 35 per cent. of insoluble phosphoric acid.  The acid phosphates or dissolved rock phosphates contain from 12 to 16 per cent. of available phosphoric acid and from 1 to 4 per cent. of insoluble.

Bone Fertilizers. Bones have long been a valuable and favored source of phosphoric acid.  In addition to phosphoric acid they contain some nitrogen which adds to their value.  They are organic phosphates and are quite lasting in their effect on the soil as they decay slowly.

The terms “Raw Bone,” “Steamed Bone,” “Ground Bone,” “Bone Meal,” “Bone Dust,” “Bone Black,” “Dissolved Bone,” indicate the processes through which the bone has passed in preparation, or the condition of the material as put on the market and used on the soil.

Ground bone, bone meal, bone dust, indicate the mechanical conditions of the bones.

The bones are sometimes ground “raw” just as they come from the slaughter-house or kitchen, or they are sometimes first “steamed” to extract the fat for soap, and the nitrogenous matter for glue.

Raw Bone. Analysis:  Nitrogen, 2.5 to 4.5 per cent.  Available phosphoric acid, 5 to 8 per cent.  Insoluble phosphoric acid 15 to 17 per cent.

Steamed Bone contains 1.5 to 2.5 per cent. of nitrogen, 6 to 9 per cent. of available phosphoric acid and 16 to 20 per cent. of insoluble phosphoric acid.

Steamed bone pulverizes much finer than raw bone and decays more rapidly in the soil because the fat has been extracted from it.

Dissolved Bone. Ground bone is sometimes treated with sulphuric acid to render the phosphoric acid in it more available.  It is then called dissolved bone and contains thirteen to fifteen per cent. of available phosphoric acid and two to three per cent. of nitrogen.

Dissolved Bone Black. Bone charcoal is used for refining sugar.  It is then turned over to the fertilizer manufacturers who sell it as “Bone Black” or treat it with sulphuric acid and then put it on the market as dissolved bone black.

The bone black contains thirty to thirty-six per cent. of insoluble phosphoric acid.

The dissolved bone black contains 15 to 17 per cent. of available phosphoric acid and 1 to 2 per cent. insoluble.

“Thomas Slag,” “Phosphate Slag,” “Odorless Phosphate.”  Phosphorous is an impurity in certain iron ores.  In the manufacture of Bessemer steel this is extracted by the use of lime which melts in the furnace, unites with the phosphorous and brings it away in the slag.  This slag is ground to a fine powder and used as a fertilizer.  It contains 11 to 23 per cent. of phosphoric acid, most of which is available.