As has already been pointed out, commercial nitrate of soda contains about 95 per cent of pure nitrate of soda, or about 15-1/2 per cent of nitrogen, which, if calculated as ammonia, would equal 19 per cent. It is, next to sulphate of ammonia (which contains 24-1/2 per cent of ammonia), the most concentrated nitrogenous manure, and further, contains its nitrogen in the form most readily available for the plant's use. Its most characteristic property is its great solubility, and consequent speedy diffusion in the soil, and the inability of the soil-particles to fix its nitrogen. In the latter respect it differs very considerably from other forms of nitrogen. Ammonia salts, though practically quite as soluble, do not diffuse in the soil so rapidly as nitrate of soda does; for the ammonia is more or less tenaciously fixed by the soil-particles, and retained till converted by the process of nitrification into nitrates.
Nitrate of Soda applied as a Top-dressing.
On this account nitrate of soda is chiefly employed—and rightly so—as a top-dressing. The risk of loss by drainage is thus minimised, and the valuable nitrogen finds its rightful destination—viz., in the plant's roots.
Encourages deep Roots.
A special benefit which the diffusibility of nitrate of soda has been held to confer on the plant, is to encourage the growth of deep roots, by inducing the growing plant to send down its roots into the lower layers of the soil after the nitrate of soda. The benefit of deep roots is, of course, very great. They enable the plant to withstand the action of drought, and at the same time increase the area whence the plant may derive its nourishment. Although the value of the manure is practically entirely due to the nitrogen it contains, it has been urged that the soda exercises a beneficial effect on the mechanical properties of the soil, by increasing its power of absorbing moisture, and in also rendering it more compact. This would partly explain how its results in dry seasons are so much better than those obtained from sulphate of ammonia. This mechanical action of nitrate can scarcely be very great when we remember the comparatively small quantity applied. Even in the driest of seasons there will always be sufficient moisture to secure the diffusion of the nitrate of soda, while the risk of loss by drainage will be reduced to a minimum. Much ignorance, as well as prejudice, has existed in the past as to the true nature of the action of nitrate of soda. Nor is this prejudice even yet entirely dispelled.
Is Nitrate an exhausting Manure?
The common charge brought against it is, that it is what has been termed an exhausting manure. This objection, to have any weight, must mean that nitrate of soda produces a crop which takes out of the soil an abnormal quantity of fertilising matter. But, so far as the writer is aware, no scientific evidence has ever been brought forward to support this contention. That the indiscriminate use of a manure may produce a crop in which the stem and leaves are unduly developed at the expense of the grain, or in which the quality of the crop may suffer from too rapid growth, is, of course, a well-known fact. But as this could also be produced by an overdose of soluble phosphoric acid as well as ammonia salts, it is not a property that belongs exclusively to nitrate of soda. Probably nitrate of soda has in the past been often used in this indiscriminate way so as to produce such results. The fault, therefore, lies not in the manure, but in the mode of its application. A few remarks, therefore, on this most important subject may prove serviceable.
Crops for which it is suited.
Opinions will naturally differ as to the crops to which it is profitable to apply nitrate of soda. Its value as a manure for cereals is pretty generally admitted. Its value as a manure for roots is not, however, so universally admitted. Experiments would seem to show that such a crop as the mangold derives just as much benefit as do the cereals; while in Germany practical experience on a very large scale has demonstrated its value as a manure for beetroots. It may be generally recommended as a manure for all crops, except, perhaps, the so-called leguminous crops, such as clover, beans, peas, &c., whose ability to obtain nitrogen for themselves renders the application of expensive artificial nitrogenous manures unadvisable.
An interesting point with regard to nitrate of soda is the curious effect it seems to have on the colour of the leaves of plants. This interesting fact has been strikingly demonstrated at the Rothamsted Experimental Station, in the contrast in the colour of the leaves of different experimental grass-plots, manured with nitrate of soda and sulphate of ammonia respectively—the plots manured with nitrate of soda being distinctly darker in hue, obviously owing to the greater production of chlorophyll or green matter. Such a depth of colour would seem to indicate a more healthy development.