esence of this dangerous impurity
is easily detected by adding ferric chloride, which, in presence of the
sulphocyanate, produces a blood-red colour. Sulphate of ammonia is thus
the most concentrated of all nitrogenous manures in common use, and is
for that reason the most expensive.

_Application._

For this reason, as well as from the fact that it contains a speedily
available form of nitrogen, sulphate of ammonia should only as a rule be
applied in comparatively small quantities--100 to 125 lb. per acre.[215]
It should also be applied before, but not too long before, the crop is
likely to require it. The reason of this is to give it time to be
converted into nitrates. The ability of the soil to retain ammonia has
already been pointed out. It is not safe, however, to rely too much on
the retentive power of the soil for ammonia, the conversion of ammonia
into nitrates going on very quickly under favourable circumstances. It
is most profitably used as a manure for cereals, and it has been found
by Lawes and Gilbert in their experiments, that an increase of one
bushel of wheat and a corresponding increase of straw have been obtained
for every 5 lb. of ammonia added to the soil. As has been pointed out in
the previous chapter, the respective merits of sulphate of ammonia and
nitrate of soda depend largely on the nature of the season during which
they are used. In wet seasons the sulphate is rather more favourable
than the nitrate, but, on an average, nitrate of soda is probably the
more valuable manure--_i.e._, due regard being had to the quantity of
nitrogen the two manures respectively contain. In one respect sulphate
of ammonia is a much more useful manure than nitrate of soda, as the
nature of its action when applied to the soil permits of it being used
as an ingredient of mixed manures.

Like nitrate of soda, but even to a greater extent, its most favourable
action is obtained when it is applied along with other manurial
ingredients. It should be applied at least a month earlier than nitrate.
It has been shown that in the case of chalky soils a certain loss of
ammonia in sulphate of ammonia is apt to take place, due to the action
of the lime; and this leads us to point out that, in preparing mixed
manures, care ought to be taken that it is not mixed with any compound
containing free lime or caustic alkali, as otherwise loss of ammonia
will ensue. It should never, for example, be used along with basic slag.

F