K₂SO₄·MgSO₄·MgCl₂·H₂O. Its theoretical percentage of potash (K₂O), oxygen = 16, is 23.2.

Pure kainit, however, is never found in commerce. It is mixed naturally as it comes from the mines with common salt, potassium chlorid, gypsum, and other bodies. The content of potash in the commercial salt is therefore only a little more than half that of the pure mineral. In general it may be taken at 12.5 per cent, of which more than one per cent is derived from the potassium chlorid present. The following analysis given by Maercker may be regarded as typical:[198]

Potassiumsulfate 21.3 per cent
Magnesium“14.5“
Magnesiumchlorid12.4“
Potassium“2.0“
Sodium“34.6“
Calciumsulfate(gypsum)1.7“
Water 12.7“
Alumina 0.8“

Kainit occurs as a crystalline, partly colorless, partly yellow-red mass. When ground, in which state it is sent into commerce, it forms a fine, gray-colored mass containing many small yellow and red fragments. It is not hygroscopic and if it become moist it is due to the excess of common salt which it contains.

According to Maercker kainit was formerly regarded as a potassium magnesium sulfate. But this conception does not even apply to the pure salt much less to that which comes from the mines. If, therefore, the agronomist desire a fertilizer free from chlorin he would be deceived in choosing kainit which may sometimes contain nearly fifty per cent of its weight of chlorids.

Where a fertilizer free of chlorin is desired, as for instance, in the culture of tobacco, kainit cannot be considered. In many other cases, however, the chlorin content of this body instead of being a detriment may prove positively advantageous, the chlorids on account of their easy diffusibility through the soil serving to distribute the other ingredients.

By reason of the presence of common salt and magnesium chlorid the ground kainit delivered to commerce tends to harden into compact masses. To prevent this in Germany it is recommended to mix it with about two and a half per cent of fine-ground dry peat.

Such a mixture is recommended in all cases where the freshly ground kainit is not to be immediately applied to the soil.

238. Carnallit.—This mineral is a mixture of even molecules of potassium and magnesium chlorids crystallized with six molecules of water. It is represented by the symbols KCl·MgCl₂·6H₂O. As it comes from the mines it contains small quantities of potassium and magnesium sulfates and small quantities of other accidental impurities. Existing as it does in immense quantities it has been extensively used for the manufacture of the commercial potassium chlorid (muriate of potash). For many purposes in agriculture, for instance, fertilizing tobacco fields, it is not suited, and it is less widely used as a fertilizer in general than its alteration product kainit. Its direct use as a fertilizer however is rapidly increasing since later experience has shown that chlorin compounds are capable of a far wider application in agriculture without danger of injury than was formerly supposed. As it comes from the mines, the Stassfurt carnallit has the following composition:[199]

Potassiumchlorid  15.5 per cent.
Magnesium“21.5““
Magnesiumsulfate12.1““
Sodiumchlorid22.4““
Calciumsulfate1.9““
Water26.1““
Undetermined0.5““