fine grinding and protects the
coarse particles from decay. It is known as bone-meal or coarse
ground-bone. A good quality of raw bone may contain 4 per cent of
nitrogen, while the phosphoric-acid content is 20 to 25 per cent. The
bones of old animals is less rich in nitrogen. The age of the animals,
and the sorting for manufactures of various kinds, cause variation in
quality, and the purchase of raw bone should be made on guaranteed
analysis just as surely as the purchase of bone that has been treated
in any way for removal of various substances in it.

Steamed Bone.--When animal bone is boiled or steamed under pressure for
removal of the fat and the cartilage, the content of nitrogen is
reduced, and the percentage of phosphoric acid is increased by this
removal of fat and nitrogenous substance. The nitrogen in steamed bone
may run as low as 1 per cent, and the phosphoric acid may go up to 30
per cent. The composition of steamed bone is so widely variable that
the name means little, and purchase should be made only on guaranteed
analysis. Some grades run very low both in nitrogen and phosphoric
acid, due probably to adulteration.

The boiling or steaming of bone makes fine grinding possible, and the
fineness and absence of fat permit quick decay in the soil. Steamed
bone is an excellent source of phosphoric acid. The availability is
less immediate than that of acid phosphate, but much greater than that
of raw bone.

Rock-phosphate.--While the greater part of our soils contain relatively
scant stores of phosphoric acid, the deposits of this plant constituent
in combination with lime are immense. The rock now chiefly used in this
country is found in South Carolina, Tennessee, and Florida. It varies
greatly in content of phosphoric acid. When pulverized for direct use
on land, without treatment with sulphuric acid to make the plant-food
available, a grade running 28 per cent phosphoric acid, or less,
usually is selected, the higher grades being reserved for treatment
with acid or for export. This untreated rock, pulverized exceedingly
fine, often is known as floats.

The value of a pound of phosphoric acid in floats, as compared with
that of a pound in the treated rock, known as acid phosphate, is a
matter upon which scientists differ widely. Only a small percentage of
the plant-food is immediately available, and the question of wise use
hinges upon the degree of availability gained later, and the time
required. The