Nitrogenous manures are chiefly valuable because they give a luxuriance and brilliancy of colour to the foliage of plants, thus enabling them under healthy conditions to absorb sufficient supplies of carbonic acid gas from the atmosphere during the daytime. The practical gardener may therefore by a mere glance at his plants be able to say whether his plants are lacking in nitrogenous food or not. When the leaves are luscious and deep green, and the shoots are gross and sappy, it is a sure sign that there is an abundance of nitrates in the soil. Such rank growth can only be produced by their presence. It would therefore be a mistake to add nitrogenous manures to such a soil. To check the rankness of growth, however, it would be wise to add phosphates, potash, or lime, and thus induce the formation of flowers and fruits instead of wood.

Amongst natural substances which supply nitrates to the soil are farmyard and stable manure, leaves, the dung of such animals as the horse, cow, pig, sheep, poultry, rabbit, and all refuse from them, such as wool, shoddy, horn, hair, feathers, skin, leather, meat meal, dried blood. To these may be added fish manure, oilcake manure, night soil, and poudrette.

Indeed these materials not only supply nitrogen, but also certain quantities of potash, phosphoric acid, and lime, as well as other foods. They may therefore be looked upon as complete fertilizers.

Amongst artificial manures supplying nitrogen are nitrate of soda, sulphate of ammonia, nitrate of potash, nitrate of lime, nitrogen, and guano.

When the natural manures, which are all of animal origin, are incorporated with the soil, and are in a thoroughly decayed condition, they possess all the advantages of humus, and are safe and reliable. They keep the temperature equable, retain sufficient moisture, bring bacteria into being, dissolve mineral matters, and gradually yield up their foods to the roots of the plants.

Nitrate Of Lime

This is a new nitrogenous fertilizer produced from the oxygen and nitrogen of the atmosphere by an electrical process. The commercial product is a hard crystalline substance which contains about 13 per cent of nitrogen. It is very soluble in water and has the disadvantage of being very deliquescent, owing to its affinity for moisture in the air. It must therefore be kept in a very dry place, and it is best used as a topdressing to growing crops in the same way as nitrate of soda.

Nitrate Of Soda of Chili Saltpetre

This is one of the best-known artificials, and enormous quantities are sold every year. It is found in layers of varying thickness in parts of Chili, whence 1,733,540 tons were exported in 1908. The Continent absorbed 1,272,000 tons, the United States 308,000 tons, and the United Kingdom 105,000 tons, about one-half the supply being used for agricultural and horticultural purposes.

Commercial nitrate of soda contains from 95 to 96 per cent of actual nitrate of soda, the remaining 4 to 5 per cent consisting of moisture, salt, soda, magnesium sulphate, etc. The best samples with 95-per-cent purity contain about 15.6 per cent of nitrogen, this being equivalent to 19 per cent of ammonia.

Nitrate of soda is a very quick-acting manure - that is, it yields up its nitrogen soon after application and especially after a shower of rain. It should therefore only be applied to soil which is carrying a crop in full growth, and which shows by the colour of its foliage that a dressing would be beneficial. As growth is most rapid in spring and summer, these are the best seasons for applying nitrate of soda. As an autumn or winter dressing it would be practically wasted. The quantity given will vary from 1 cwt. to 2 cwt. per acre, or, roughly, 3/4 lb. to 1 1/2 lb. to every square rod or pole of ground. As a stimulant in conjunction with organic manures already in the soil, nitrate of soda is excellent for Cabbage crops, including Turnips and Kohl Rabi, as well as for Beet, Spinach, etc. It is only rarely necessary to apply it to leguminous crops like Peas and Beans, as these are capable of securing their own supplies of nitrogen from the atmosphere. Perhaps the best way to use nitrate of soda is as a topdressing, afterwards working it into the soil with the hoe; or for pot plants by dissolving about 1 oz. in 1 gal. of water. If used dry, a mere pinch - as much as will cover a threepenny piece - is quite sufficient for plants in 5-in. pots.

Nitrate of soda may be used with basic slag, but should never be mixed with sulphate of ammonia or kainit; and it can be only safely mixed in small quantities with superphosphate of lime, owing to the danger of decomposition.