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 a whitish crystalline salt manufactured from natural deposits in the German potash mines. As a manure, the best samples contain 98 per cent of sulphate of potash, equal to over 52 per cent of pure potash. Inferior samples contain about 90 per cent of sulphate of potash, equal to 48 per cent of pure potash. The double sulphate of potash magnesia, which contains a good deal of magnesium sulphate, is often called sulphate of potash, but it is inferior in potassic value. It contains about 50 per cent of sulphate of potash, equal to 27 per cent of pure potash.