SULPHATE OF AMMONIA.
_Value of Ammonia as a Manure._
The value of ammonia salts as a manure has been long recognised; indeed
till recently ammonia was thought to be the most valuable form in which
nitrogen could be applied as a plant-food--a view, we may mention, held
by Liebig. While the plant, no doubt, can absorb its nitrogen in the
form of ammonia,[212] as well as in other forms, as we have already
pointed out in previous chapters, it is now fully recognised that
ammonia salts, when applied to the soil, are converted into nitrates.
Nitric acid, then, must be regarded as the most valuable, inasmuch as it
is the most rapidly assimilated form of nitrogen for the plant; but
next to nitric acid in value comes ammonia. Of the different forms of
ammonia available for manurial purposes, the only one used to a large
extent is sulphate.
_Sources of Sulphate of Ammonia._
The oldest, and what is still the chief source of this valuable salt, is
the gas-works, where it is obtained as one of the bye-products in the
manufacture of gas. It is also obtained to a lesser extent from shale,
iron, coke, and carbonising works. Bones, horn, leather, and certain
other animal substances rich in nitrogen, when subjected to dry
distillation, as is the case in certain manufactures, such as the
manufacture of bone-charcoal for use in sugar-refineries, and the
distillation of horn, &c., in the manufacture of prussiate of potash,
also constitute less abundant sources.
_Ammonia from Gas-works._
Coal contains on an average from a half to one and a half per cent of
nitrogen. When it is subjected to dry distillation, as is done in the
gas-works, the nitrogen which it contains is chiefly converted into
ammonia, and, in the process of purification of the gas, is removed in
the "gas-liquor,"[213] which contains about one per cent of ammonia.
The ammonia recovered from this liquor by distillation is then absorbed
in sulphuric acid. It may be pointed out that nothing like all the
nitrogen contained in the coal is recovered as sulphate of ammonia. It
has been calculated that only from a fifth to a tenth is actually
recovered, and many processes have been patented with a view to
increasing the yield of ammonia in gas manufacture. The total production
of ammonia from gas-works may be placed at little over 100,000 tons per
annum for Great Britain. Mr L. Mond, F.R.S., recently drew attention to
the possibility of largely increasing our supply of sulphate of ammonia
from coal. As indicating what an enormous source of sulphate of ammonia
we have in coal, Mr Mond calculated that its annual consumption in this
country (estimated at 150,000,000 tons) would yield as much as 5,000,000
tons of sulphate of ammonia.
_Other Sources._
While the ammonia produced in the manufacture of gas has long been
collected, it is only of recent years that the other sources of ammonia
have been developed. Next to the gas-works, the shale-works of Scotland
form in this country the chief source of this valuable manure. In these
works the ammonia is obtained in distilling the paraffin shale by a
method somewhat similar to that in use in the gas-works. The amount of
sulphate of ammonia obtained from this source is between 20,000 and
30,000 tons per annum. Recently the ammonia has been recovered from the
blast-furnace gases in iron-works--some 6000 tons being annually
obtained in this way; while from coke and carbonising works the annual
production is about half that amount. The combined annual production
from all these sources may be put down at 140,000 tons, the total
production in Europe being probably little more than 200,000 tons. In
the Appendix further statistics will be found.[214]
_Composition, &c., of Sulphate of Ammonia._
Pure sulphate of ammonia is a whitish crystalline salt, extremely
soluble in water. The commercial article, however, is generally greyish
or brownish in colour, owing to the presence of slight quantities of
impurities. The pure salt should contain 25.75 per cent of ammonia; but
the commercial article is generally sold on a basis of 24.5 per cent. A
useful test of its purity is the fact that when subjected to a red-heat
it should almost entirely volatilise, leaving very little residue. The
chief impurities which it is likely to contain are an excess of
moisture, free acid, or the presence of insoluble matter. Certain
samples contain small quantities of ammonium sulphocyanate, an extremely
poisonous substance for plants. The presence 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.
FOOTNOTES:
[212] From experiments by Lehmann and others with buckwheat and maize,
it would seem that certain plants may prefer, at certain stages of their
growth, ammonia to nitrates. In the case of maize, ammonia may be
preferred in the early stages of growth, while nitrates are preferred as
it becomes more mature. In view, however, of our present knowledge of
nitrification, it may well be doubted whether the conclusions arrived at
from Lehmann's experiments can be accepted.
[213] As the expense of converting the ammonia present in the ammoniacal
liquor is considerable, the practice of using the liquor itself as a
manure has been advocated; but as an objection to this it must be urged
that, besides being so bulky a manure, the liquor contains various
substances poisonous to plant-life.
[214] See Appendix, p. 358.
[215] Some crops, however, may with advantage be treated with larger
quantities of sulphate of ammonia, such as mangels and potatoes.
APPENDIX TO CHAPTER X.
NOTE (p. 355).
The following table will exhibit the production of sulphate of ammonia
in this country from 1870 to 1892:--
Year. Tons. | Year. Tons.
1870 40,000 | 1882 72,000
1871 41,000 | 1883 75,000
1872 42,000 | 1884 87,000
1873 43,000 | 1885 97,000
1874 45,000 | 1886 106,500
1875 46,000 | 1887 113,700
1876 48,000 | 1888 122,800
1877 52,000 | 1889 132,000
1878 55,000 | 1890 140,000
1879 57,000 | 1891 143,500
1880 60,000 | 1892 157,000
1881 65,000
The following table exhibits the sources, and the respective quantities
from each source, of the last seven years' production:--
1886. 1887. 1888. 1889. 1890. 1891. 1892.
Gas-works 82,500 85,000 93,000 100,000 102,150 107,950 112,000
Iron-works 4,000 5,000 5,300 6,000 5,050 6,300 12,000
Shale-works 18,000 21,000 22,000 23,000 24,750 26,600 28,000
Coke and
carbonising
works 2,000 2,700 2,500 3,000 2,300 2,800 5,000