These, with few exceptions, are natural products from the potash mines
of Stassfurt (Prussia). Until the discovery of these deposits, in 1861,
the use of potash as a fertilizing constituent was very limited, being
confined practically to the employment of wood ashes. At the present
time a small quantity of potash salts--principally carbonate of
potash--is obtained from sugar refinery and other manufacturing
processes, but the great bulk of the potash supply comes from the German
mines. In these the different natural salts occur in different layers
and in conjunction with layers of rock-salt, carbonate of lime and other
minerals, from which they have to be separated out and undergo
subsequently a partial purification by re-crystallization.
The principal potash salts used in agriculture are--(1) sulphate of
potash, which is about 90% pure; (2) kainit, an impure form of
sulphate of potash, and containing much common salt and magnesia
salts, and giving about 12% of potash (K2O); (3) muriate of potash,
which is used to a great extent in agriculture, and contains 75 to 90%
of muriate of potash; and (4) potash manure salts, a mixture of
different salts and containing from 20 to 30% of potash.
Potash is much esteemed in agriculture, more especially on light land
(which is frequently deficient in it) and on peaty soils, and for use
with root crops and potatoes in particular. For fruit and vegetable
growing and for flowers potash manures are in constant request. Clay
land, as a rule, is not benefited by their use, these soils containing
generally an abundance of potash. Along with basic slag, potash salts
have been frequently used for grass on light land with advantage.