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.