house waste, such as meat and bone
scrap, are boiled or steamed to extract the fat. The settlings are
dried and ground and sold as tankage. It is much slower in its action
than dried blood and supplies the crop with both nitrogen and
phosphoric acid.

_Dried Fish Scrap_ is a by-product of the fish oil factories and the
fish canning factories. It contains 7 to 9 per cent. of nitrogen and 6
to 8 per cent. of phosphoric acid. It undergoes nitrification readily
and is a quick acting organic source of nitrogen and phosphoric acid.

_Cotton-seed Meal_ contains 7 per cent. of nitrogen, about 2.5
phosphoric acid, and 1.5 per cent. of potash. It is a product of the
cotton oil factories and is obtained by grinding the cotton seed cake
from which the oil has been pressed. It is a most valuable source of
nitrogen for the South.

The nitrogen in the dried blood, tankage, fish scrap and cotton-seed
meal, being organic nitrogen, must be changed by the process of
nitrification to nitric acid or nitrate before it is available. They
are therefore better materials to use for a more gradual and
continuous feeding of crops than the nitrate of soda or sulphate of
ammonia.

Scrap leather, wool waste, horn and hoof shavings are rich in nitrogen
but they decay so slowly that they make poor fertilizers. They are
used by fertilizer manufacturers in making cheap mixed fertilizers.

SOURCES OF PHOSPHORIC ACID

The principal commercial sources of phosphoric acid are:

Phosphate Rocks.
Bones.
Fish scrap.
Phosphate slag.

The _Phosphate Rocks_ are found in shallow mines in North and South
Carolina, Georgia, Florida and Tennessee, and also as pebbles in the
river beds. They are the fossil remains of animals. After being dug
from the mines the rock is kiln dried and then ground to a very fine
powder called "floats" which is used on the soil. The phosphoric acid
in the floats is insoluble and becomes available only as the phosphate
decays. This is too slow for most plants so it is treated with oil of
vitriol or sulphuric acid to make it available. The phosphoric acid in
the ground rock is combined with lime, forming a phosphate of lime
which is insoluble. When treated with the oil of vitriol or sulphuric
acid, the sulphuric acid takes lime from the phosphate and forms
sulphate of lime or gypsum. The phosphoric acid is left combined with
the smallest possible amount of lime and is soluble in water. It is
then called soluble or water