potash, it contains nearly all the other plant ingredients,
such as lime, magnesia, &c. Its special value as a manure, however, does
not merely consist in the amount of valuable plant-food it contains.
Like farmyard manure, it owes much of its characteristic action to the
state of the intimate mixture of its manurial constituents, and also, as
has already been pointed out, to the fact that it contains those
constituents in a great variety of chemical forms, each of which differs
in its solubility, and consequently availability for the plant's needs.
Take, for example, the great number of different forms of nitrogen it
contains. Some are in the condition in which plants can immediately
absorb them, while the rest are in a series of less and less available
forms, which, however, are gradually converted into available forms as
the plant requires them. Like farmyard manure, again, it may be applied
with almost equally good results to all kinds of crops and on all kinds
of soils. We have in guano, in short, an admirable example of the value
of applying our manurial ingredients in different forms. That this is no
mere theory is abundantly proved by the large number of different
experiments which have in the past been carried out with guano, more
especially the well-known experiments made by Grouven, the German
chemist. In those well-known experiments, guano was tested against a
large variety of different fertilisers, and the tests were so arranged
that in most cases the amounts of nitrogen, phosphoric acid, and potash
were the same in the other manures used. In short, these experiments
prove in a very striking manner that a manure artificially made up out
of most valuable fertilisers, such as nitrate of soda, sulphate of
ammonia, superphosphate, &c., so as to closely resemble in its
composition guano, is by no means similar in its effects to the genuine
article. As in farmyard manure, so in guano: we must look to the
complexity of the composition of both these fertilisers in order to
fully estimate their worth. There is in the action of both manures much
that we cannot explain, or even, as yet, understand. The action of guano
is merely one of many problems in the science of manuring which
illustrate how unsatisfactory, despite the great amount of research
already carried out, is our knowledge of this most important department
of agriculture.[197]

_Proportion of fertilising Constituents in Guano._

Guano must be regarded