soluble phosphoric acid.

Now if this soluble form remains unused it begins to take on lime
again and turns back toward its original insoluble form. After a time
it gets to such a state that it is no longer soluble in water but is
soluble in weak acids. It is then said to be reverted phosphoric acid.
Reverted phosphoric acid is also called citrate soluble phosphoric
acid, because in testing fertilizers the chemists use ammonium citrate
to determine the amount of reverted phosphoric acid.

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