s. Mangel and plants of the cabbage tribe may
be cited as examples of plants containing large amounts of soda in their
composition. But the plants which contain it in largest quantity are
those which thrive on the sea-coast, and it has been thought that for
them at least salt is a necessary manure. This, however, does not seem
to be the case. In fact, the amount of soda in a plant seems to be
largely a matter of accident. It may be added that the succulent
portions of a plant are generally richest in soda.

_Can Soda replace Potash?_

Again, it has been believed that soda is capable of replacing potash in
the plant; but this does not seem to be the case to any extent. The view
that soda is able to replace potash, it has been thought, is supported
by the variation which exists in the proportion of soda and potash in
different plants. It must be remembered, however, that it is highly
probable that most plants contain a larger quantity of ash-constituents
than is absolutely necessary for their healthy growth. Especially is
this the case with such a necessary plant-food as potash, of which there
is generally present, in all likelihood, an excess. The variation in the
quantity of potash and soda present in many plants under different
circumstances can scarcely, therefore, be regarded as furnishing a proof
of the replacement of potash by soda. Incidentally we may mention, as a
fact worthy of notice, that cultivated plants have more potash and less
soda in their composition than wild plants. What has been said of soda
may be held to apply equally to chlorine, as it seems to be chiefly in
the form of common salt that soda enters the plant. The amount of salt,
therefore, present in plants must be regarded as largely accidental and
dependent on external circumstances, such as the nature of the soil, &c.

_Salt of universal Occurrence._

But even were salt a necessary plant-food, its occurrence in the soil is
already of sufficient abundance to obviate any necessity for its
application. It may be said to be of almost universal occurrence. Even
the air contains it in traces. That this is the case in the
neighbourhood of the sea-coast is well known; but even in air far
inland, accurate analysis of the air would probably demonstrate its
presence in greater quantity than is commonly believed. It is a wise
provision that plants absorb salt, for it increases their efficiency as
food,--the function of salt as a constituent of anima