re when applied to the soil. Bone-meal of good
quality contains from 45 to 55[217] per cent of phosphate of lime, and
3-1/2 per cent of nitrogen. Our present total consumption of bones is
probably little less than 100,000 tons per annum, of which about half is
obtained from home collections, over 20,000 tons being annually
collected in and around London alone.

_Action of Bones._

It is well known that bones are a slow-acting manure. They may be said
to possess both a mechanical and chemical action when applied to the
soil. When they putrefy, their nitrogen is slowly converted into
ammonia, and carbonic acid as well as various organic acids are formed,
which, acting upon the insoluble mineral matter in the bones, renders it
available for plant uses. Bones thus, when applied in large quantities,
may not merely act directly as suppliers of plant-food, but in the
course of their putrefaction may act upon a certain amount of the inert
fertilising matter of the soil and render it available. The more
readily, then, bones putrefy, the more speedy will be their effect. As
we have already pointed out, bones, in order to increase their
efficiency, are often fermented before application. The removal of the
fat is another means of increasing the rate of their action, but the
fineness to which they are ground determines this more than anything
else. Much ingenuity has been expended in perfecting machinery for
grinding bones. At one time in Germany they were pounded in stamps
similar to those used for ore. In America what has been called "floated
bone" has been prepared. This bone is so fine that it actually floats in
the air like flour-dust, and is made by whirling the bones against one
another. The action of bones prepared in this way is of course very
speedy, but the difficulty of applying a manure in such a fine state of
division to the soil is great. The expense of the process also is
considerable.

The ease with which bones when ground into a fine state of division
putrefy, is evidenced by the fact that bone-flour has to be salted in
order to enable it to keep. Another condition which determines the rate
at which the fertilising matters in bones become available is the nature
of the soil. Fermentation, as we have already seen, requires a plentiful
supply of air, and a certain amount, but not too much, of moisture.
Consequently bones act best in medium soils--soils which are "neither
too light and dry, nor too close and wet