he extent of the nitrate of soda deposits is naturally
one of very great interest, especially from the agricultural point of
view. M. Charles Legrange, a French writer, estimated a few years ago
that they still contained about 100,000,000 tons of pure nitrate of
soda. Opinions on this point differ very considerably, and it seems
wellnigh impossible to arrive at any very accurate estimate.

The number of years they will last will depend, of course, on the amount
of annual exportation. This, at present, falls little short of 1,000,000
tons. If this amount is maintained, they should last, according to
experts, some twenty or thirty years at least. A consideration which has
an important influence on this question, is the price obtained for the
article. If this should be increased, it may be possible to treat the
larger quantities of the inferior raw material (which at present prices
are allowed to accumulate) at a profit. Undoubtedly this is what will
ultimately take place, when the richer quality of the _caliche_ has been
exhausted.

_Composition and Properties of Nitrate of Soda._

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