in the
section on soils, but it may be mentioned here that accurate experiment
has shown that a good peat will absorb about 2 per cent[L] of ammonia,
and when dry will still retain from 1 to 1.5 per cent, or nearly twice
as much as would be yielded by the whole nitrogen of an equal weight of
farm-yard manure. Peat charcoal has been recommended for the same
purpose, but careful experiment has shown that it _does not absorb
ammonia_, although it removes putrid odour; and though it may be
usefully employed when it is wished to deodorize the manure heap, it
must not be trusted to for fixing the ammonia.

Much stress has frequently been laid on the advantage to be derived from
the use of substances capable of combining chemically with the ammonia
produced during the fermentation of dung and gypsum, sulphate of iron,
chloride of manganese, sulphate of magnesia, and sulphuric acid, have
been proposed for this purpose, and have been used occasionally, though
not extensively. They all answer the purpose of _fixing_ the ammonia,
that is, of preventing its escaping into the air; but the risk of loss
in this way appears to have been much exaggerated, for a delicate
test-paper, held over a manure-heap, is not affected; and during
fermentation, humic acid is produced in such abundance, as to combine
with the greater part of the ammonia. The real source of deterioration
is the escape of the soluble matters in the drainings from the
manure-heap, which is not prevented by any of these substances; and
where no means are taken to preserve or retain this portion, the loss is
extremely large, and amounts, under ordinary circumstances, to from a
third to a half of the whole value of the manure. Manure, therefore,
cannot be exposed to the weather without losing a proportion of its
valuable matters, depending upon the quantity of rain which falls upon
it. Hence it is obvious that great advantage must be derived, especially
in rainy districts, from covered manure-pits. This plan has been
introduced on some farms with good effect; but the expense and doubts
as to the benefits derived from it, have hitherto prevented the practice
becoming general. The principal difficulty experienced in the use of the
covered dung-pit is, that, where the litter is abundant, the urine does
not supply a sufficiency of moisture to promote the active fermentation
of the dung, and it becomes necessary to pump water over it at
intervals; but when this is properly done