soils rich in this ingredient.

_Stassfurt Salts._

But in addition to the sources of potash already mentioned, it exists in
other forms in the earth's surface. Till within recent years it was
obtained for commercial purposes from the ashes of plants, which, as we
shall immediately see, are extremely rich in this ingredient; from salt
water--this source giving rise to the so-called "salt gardens" on the
coast of France; and from nitre soils in different parts of India,
referred to already at considerable length. Large mineral deposits,
however, have been recently discovered in the neighbourhood of Stassfurt
in Germany, and have since their discovery supplied all the potash
required for manurial and other purposes. In these deposits (similar
ones have also been found at Kalusz in the Carpathian Mountains) there
are no less than five different minerals which contain potash. The form
in which it is present is as sulphate or chloride, so that it is readily
available for plants, and is of altogether very much greater value than
the form in which it occurs in the minerals already mentioned--viz., as
an insoluble silicate. Of the Stassfurt potash salts, the best known as
a manure is _kainit_, which contains about 32 per cent of sulphate of
potash. A list of the other potash minerals, with the particulars of
their composition and the percentage of potash they contain, will be
found in the Appendix.[128]

_Occurrence of Saltpetre._

We have already had occasion, in Chapter IV., when discussing the
question of nitrification, to refer to the occurrence of nitrate of
potash in certain soils in India, which have formed a large source of
saltpetre used in commerce in the past.

_Occurrence of Potash in the Soil._

From what has been said regarding the richness in potash of certain
commonly occurring minerals, such as felspar, it is only natural to
infer that most soils must contain large quantities of this substance;
and this is so. The wonder is that potash, when applied as an artificial
manure, should have such a marked effect in increasing the fertility of
the soil, as is often the case. We must remember, however, that although
a soil may contain large quantities of potash, there may be a very small
percentage of the whole in an available form for the plant's needs.

_Potash chiefly in insoluble Condition in Soils._

Potash occurs almost entirely in soils in a very insoluble form--viz.,
combined with silica as a