These may be classified as follows: (a) Natural manures--bones, fish and
meat guanos, Peruvian guano, bats' guano; (b) Manufactured
manures--dissolved bones, compound manures.
a. _Natural Manures_
_Bones.._--The value and use of these in agriculture has long been
known, as also the comparative slowness of their action, which latter
induced Liebig to suggest their treatment with sulphuric acid. Natural
bones will contain from 45 to 50% of phosphate of lime with 4 to 4½%
of nitrogen. It is usual to boil bones lightly after collection, in
order to remove the adhering particles of flesh and the fat. If
steamed under pressure the nitrogenous matter is to a great extent
extracted, yielding glue, size, gelatine, &c., and the bones--known
then in agriculture as "steamed bones"--will contain from 55 to 60% of
phosphate of lime with 1 to 1½% of nitrogen. Bones are also imported
from India, and these are of a very hard and dry nature. Bones are
principally used for root crops, and to some extent on grass-land. The
more finely they are ground the quicker is their action, but they are
a slow-acting manure, which remains some years in the land. Mixed with
superphosphate, bone meal forms an excellent manure for roots, and
obviates the difficulty of using superphosphate on land poor in lime.
Steamed bones, sometimes ground into flour, are much used in dairy
pastures.
_Fish and Meat Guanos._--The term "guano," though generally applied to
these manures, is wrongly so used, for they are in no sense guano
(meaning thereby the droppings of sea birds). They are really fish or
meat refuse, being generally the dried fish-offal or the residue from
meat-extract manufacture. They vary much in composition, according to
their origin, some being highly nitrogenous (11 to 12% nitrogen) and
comparatively low in phosphate of lime, and others being more highly
phosphatic (30 to 40% phosphate of lime) with lower nitrogen. These
materials are to some extent used for root and vegetable crops, and
chiefly for hop-growing, but they go largely also to the artificial
manure maker.
_Peruvian Guano._--This material, though once a name to conjure with,
has now not much more than an academic interest, owing to the rapid
exhaustion of the supplies. It is true guano, i.e. the deposit of sea
birds, and was originally found on islands off the coast of Peru.
Peruvian guano was first discovered in 1804 by A. von Humboldt, and
the wonderful results attending its use gave an enormous impulse to
its exportation. The Chincha Islands yielded the finest qualities of
guano, this giving up to 14 and 15% of nitrogen. Gradually the Chincha
Islands deposits became worked out, and other sources, such as the
Pabellon de Pica, Lobos, Guanape and Huanillos deposits were worked in
turn. In many instances the guano had suffered from washing by rain or
by decomposition, or in other cases the bare rock was reached and the
shipments contained some considerable quantity of this rocky matter,
so that the highly nitrogenous guanos were no longer forthcoming and
deposits more phosphatic in character took their place. Gradually the
shipments fell off, and with them the great reputation of the guano as
a manure. On some of the islands the birds, after having been driven
off, have returned and fresh deposits are being formed. On the west
coast of Africa also some new deposits have been found, and a certain
amount of guano comes from Ichaboe Island; but the trade will never be
what it once was. Occasional shipments come from the Ballista Islands,
giving from 10 to 11% of nitrogen with 11 to 12% of phosphoric acid,
and lower-grade guanos (7% of nitrogen and 16% of phosphoric acid) are
arriving from Guanape, while from Lobos de Tierra comes a still lower
grade.
The particular feature that marked guano was that it contained both
its nitrogenous and phosphatic ingredients in forms in which they
could be very readily assimilated by plants. Moreover, the occurrence
of the nitrogenous and phosphatic matters in different forms of
combination gave to them a special value, and one that could not be
exactly imitated in artificial manures. The nitrogenous matters, e.g.,
exist as urates, carbonates, oxalates and phosphates of ammonia, and a
particular nitrogenous body termed "guanine" is also found. Guano
contains much alkaline salts, and is, from its containing alike
phosphates, nitrogen and potash in suitable forms and quantity, an
exceedingly well balanced manure. In agriculture it is used for corn
crops, and also for root crops, potatoes and hops. It is esteemed for
barley, as tending to produce good quality. For vegetable and
market-garden crops that require forcing guano is also still in
demand. The more phosphatic kinds are sometimes treated with sulphuric
acid, and constitute "Dissolved Peruvian Guano."
_Bats' Guano._--In caves in New Zealand, parts of America, South
Africa and elsewhere, are found deposits formed by bats, and these are
used to some extent as a manure, though they have no great commercial
value.
b. _Manufactured Manures_
_Dissolved Bones._--These are bones treated with oil of vitriol, as in
superphosphate manufacture. By this treatment bones become much more
readily available, and are used to a considerable extent, more
especially for root crops. Their composition varies with the method of
manufacture and the extent to which they are dissolved. Speaking
generally, they will have from 11 to 19% of soluble phosphate, with 20
to 24% of insoluble phosphates, and if pure should contain 3% of
nitrogen. When mixed with superphosphate in varying amount, or if made
with steamed and not raw bone, they are generally known under the
indefinite name of "bone manure."
_Compound Manures._--To this class belong the manures of every
description which it is the aim of the artificial manure manufacturer
to compound for particular purposes or to suit particular soils or
crops. The base of all these is, as a rule, mineral superphosphate or
else dissolved bones, or the two together, and with these are mixed
numerous different manurial substances calculated to supply definite
amounts of nitrogen, potash, &c. Such manures, the trade in which is a
very large one, are variously known as "corn manure," "turnip manure,"
"grass manure" and the like, and much care is bestowed on their
compounding and on their preparation in good condition to allow of
their ready distribution over the land.