in the soil, as well as from the fact that
under the ordinary conditions of agriculture, although removed from the
soil in considerable quantities by crops, it finds its way back again in
the farmyard manure; for it has not the same tendency to accumulate in
large quantities in the grain or seed as we have seen to be the case
with phosphoric acid. On this account straw contains a much greater
proportion of potash than phosphoric acid, and hence farmyard manure may
be regarded as fairly rich in potash.

_Occurrence of Potash._

Of all sources of potash the ocean must be regarded as the chief.
Millions and millions of tons are present in a state of solution in the
salt water of the ocean.[126] Like phosphoric acid, its occurrence in
the rocks forming the earth's crust may be said to be practically
universal. Many of the commonly occurring rocks and minerals are
extremely rich in it, and by their disintegration furnish large
quantities to the soil. Some of these rocks contain it in such abundance
that they have been tried as potash manures; and were other more
valuable sources less available than they actually are, such a practice
might well be recommended. A volcanic rock known as _palagonite_, and
that most commonly occurring of all potash minerals--viz., felspar--have
both been experimented with in this way with considerable success.

_Felspar and other Potash Minerals._

That felspar should prove, when finally ground, a valuable source of
potash, is not to be wondered at when we remember that some varieties of
it contain over 16 per cent. It has been calculated that a single cubic
foot of this mineral is sufficient to supply an oak-wood, covering a
surface of 26,910 square feet, with potash for a period of no less than
five years.[127] Some idea of the enormous _potential_ fertility of a
soil containing felspar, so far as potash is concerned, may be obtained
from this statement. It must be remembered, however, that it is only the
orthoclase or potash felspars which contain large quantities of
potash--other felspathic rocks, such as oligoclase and labradorite,
being comparatively poor in it. Another commonly occurring mineral which
is rich in potash is mica, which has been found to contain from 5 to 13
per cent. From this it follows that rocks which have large amounts of
these minerals in their composition--such as granite, for example, which
often contains 5 or 6 per cent of potash--form by their disintegration