be recognised from the
favourable action of wood-ashes. Of course their favourable action is
not due solely to potash, as they contain, in addition to the other ash
ingredients of the plant, phosphates; and their value as a manure may
also be said to depend not a little on their indirect action. They
contain a certain percentage of caustic alkali, which promotes the
decomposition of the nitrogenous matter of the soil. But making due
allowance for these other valuable properties, the chief value of
wood-ashes is undoubtedly due to the potash they contain. Hence the use
of the commercial article called _potash_, which is a mixture of
potassium carbonate and hydrate, and which is obtained from wood-ashes,
was formerly common to a considerable extent as a manure, especially for
clover. _Barilla_, a rich potassic manure prepared by burning certain
strand plants, especially the saltwort, was also in the past largely
exported from Sicily and Spain. _Kelp_, a product got by burning
sea-weed in Scotland, is also a rich potassic manure. Since, however,
the discovery of the Stassfurt mines, all potassic manures have come
from these.

_Stassfurt Potash Salts._

Huge salt deposits exist at Stassfurt in Germany. They have been formed
by the evaporation of an inland sea. Salt was first discovered in these
deposits in 1839, but for long the presence of potash salts was little
suspected, and it was not until 1862 that the potash salts were worked.
We have already, in the Appendix to Chapter VI., given a list of the
chief potash minerals occurring in the Stassfurt deposits. These
minerals are found in layers, the lowest layer consisting of almost pure
salt; while immediately above this we have a layer of salt mixed with
the mineral polyhallite (containing potassium sulphate) of about 100
feet thick. Above this last layer there is a layer of about 90 feet,
containing kieserite (magnesium sulphate) mixed with potassium and
magnesium chlorides; and above this again is a layer (90 feet) of
carnallite, which furnishes the chief source of the potash salts used
for manurial purposes.

At first the crude salts, as obtained direct from the deposits, were
sold as manures under the name of _Abraum_ salts. Now, however, they are
purified. Of potash salts in 1888 some 25,000 tons were exported from
Stassfurt for manurial purposes. Of these salts there may be mentioned,
viz., kainit, an impure form of the sulphate, containing on an average
abou