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?_