of uniform composition. The custom, first introduced
in 1864 by Messrs Ohlendorff & Co., was soon largely practised. The
guano is treated with 25 to 30 per cent sulphuric acid (sp. gr. 1.73).
After a short time the resulting hard mass is, by means of
disintegrators, reduced to a uniform powder.

_"Equalised" or "Rectified" Guano._

As guano decreased in its quality the demand for a high-class article
became more and more difficult to meet. This led to the custom of
"fortifying" or "rectifying"--as it is variously called--the natural
material with sulphate of ammonia. A manure closely resembling in the
percentage of its manurial constituents the older rich guanos is thus
obtained. Of these so-called "equalised" guanos, two qualities are at
present sold, the first being guaranteed to contain nitrogen equal to 8
to 9 per cent ammonia, 30 to 35 per cent phosphates, and 2 to 3 per cent
of potash; the second quality containing only about half as much
nitrogen, but more phosphates.

However valuable this fortified guano may be--and it is, undoubtedly, a
most valuable manure--its action cannot be supposed to be exactly
similar to the old Peruvian guano, which it resembles in the percentage
of its nitrogen, phosphates, and potash. Much of the distinctive value
of guano as a manure, as will be pointed out immediately, lies in the
fact that it contains its manurial ingredients in a variety of
differently soluble compounds, which are gradually rendered available in
the soil for the plant's needs. This undoubtedly is one of the reasons
why the action of guano among manures is quite unique; and there are
other reasons which we probably do not clearly understand. However
skilfully the composition of the guano may be artificially simulated, it
still remains an undoubted fact that the "equalised" guano is not
exactly similar in its action to the genuine article. Nevertheless, that
it is superior in its results to the poorer classes of guano at present
available, and to ordinary compound manures, there can be little doubt.
A great merit of the equalised guano is, however, that it is sold at a
lower price than guano as imported; and as the guano is sold on a
guaranteed analysis, the practice has done much to advance the true
interests of agriculture.

_Its Action as a Manure._

Next to farmyard manure, guano may be regarded as the most "general" of
all the commonly used manures; for in addition to nitrogen, phosphoric
acid, and