Now I've read something which makes me wonder. One source (the T H Everett Encyclopedia of Gardening from the 1960s) makes the claim that muriate of potash is 85% potassium chloride and 15% sodium chloride. You mean it is a mixture of Muriate of potash and Muriate of soda. Many agricultural chemicals were, and still are to some extent, quite impure. They are much cheaper than pure chemicals and often it does not matter too much. I still see chemicals that should be pure white that are quite grey or full of black specks etc. I am still trying to remember what the salts of Oil of Vitriol (sulphuric acid, or sulfuric if you prefer) were called, before sulphate. Brian Whyer, Buckinghamshire, England, zone COLD still January temperatures here.