n in a manure is actually
present in the form of ammonia. Thus, for example, when it is stated in
an analysis of bone-meal that it contains 3.5 per cent of nitrogen,
equal to 4.20 per cent of ammonia, it is not to be inferred that
bone-meal actually contains nitrogen in the form of ammonia. In point of
fact the nitrogen is present in an insoluble, slowly available, organic
form, which possesses a manurial value very inferior to that possessed
by ammonia. This custom is a most unfortunate one, and is much to be
regretted, as it is often liable to give rise to serious
misunderstanding. It must be remembered, therefore, that an ordinary
chemical analysis does not always specify the exact form in which
nitrogen is actually present. It is nevertheless of importance for the
farmer to know this, of which the nature of the manure analysed is
generally a good indication. Unfortunately this is not shown in the case
of _mixed_ manures; and this constitutes one of the reasons why mixed
manures are sometimes to be regarded with suspicion.

_Phosphoric Acid._

The amount of phosphates present in a manure is usually stated in its
analysis as so much phosphoric acid, while in a footnote the quantity of
tricalcic (or ordinary bone) phosphate this amount is equivalent to is
also given, this being the unit of valuation. When the phosphates are in
a soluble condition they are stated as such, and at the same time a
statement is made as to the quantity of tricalcic phosphate which would
be required to furnish this amount by treatment with sulphuric acid.
Thus, for example, in an analysis of a superphosphate of lime, the
statement, _monocalcic phosphate, 17.3 per cent, equal to tricalcic
phosphate rendered "soluble," 27.2 per cent_, means that it would
require 27.2 per cent of tricalcic phosphate to furnish 17.3 per cent of
soluble phosphate. Paradoxically enough, the former amount is called
_"soluble" phosphate_, and such a superphosphate as the above would be
described as containing 27.2 per cent of "soluble" phosphate.

Again, there are different forms of the so-called "insoluble"
phosphates,[248] although they are often not distinguished in a chemical
analysis. As we have already pointed out in the chapter on Basic Slag,
phosphoric acid occurs in the slag in the form of tetrabasic phosphate
of lime, although it is invariably stated in analysis as so much
tricalcic phosphate. Then we have the so-called dibasic phosphate of
lime, the f