This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is one of the most popular potash manures at present in use. It is a crude natural salt obtained from Germany, and varies in colour from creamy white to pale pink. Pure samples contain potassium equal to nearly 19 per cent of potash. The usual commercial article only contains about 124 per cent of potash. In bulk, commercial kainit contains about 35 per cent of common salt, about 30 per cent of magnesium salts (chiefly Epsom salts), and about 12.5 per cent of water of crystallization. The remainder - 22.5 per cent - is almost entirely potassium salts. It will thus be seen that less than one-fourth the bulk consists of the important fertilizer potash.
As a manure, kainit has the disadvantage of having such a large percentage (35) of common salt, but with such crops as Mangels and Asparagus this is not a drawback, as those crops benefit by the addition of salt to the soil. It would, however, be unwise to apply kainit to a soil carrying growing crops, as the salt and other impurities are likely to injure the tender rootlets.
The best time to apply kainit is a few weeks before the crop is to be sown or planted. The salt in it, being readily soluble, will be washed down into the soil out of reach of the roots, and the potash will be left behind evenly distributed amongst the soil particles. From 2 to 4 cwt. per acre is a fair dressing for kainit.