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.

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