Potash is a white or greyish material, usually sold as a powder or as small hard lumps, that farmers put into soil to help plants grow. It is one of the three things farmers add most often — nitrogen, phosphate, and potash — and it is the potassium one. Plants need potassium to build strong stems, to fight off disease, and to make good fruit and grain, so a field that has been used for many years often runs low on it and the farmer buys potash to put it back.

A note on the word: 'potash' is a farming and industry word, not really a chemistry word. A chemist would name the exact compound (potassium carbonate, potassium chloride, and so on). 'Potash' covers a whole family of potassium salts loosely, and which one it means depends on who is talking. In the old days it usually meant potassium carbonate, made by burning wood. Today, when people talk about mining potash or the price of potash, they almost always mean potassium chloride dug out of the ground. So the same word has quietly changed what it points to.

You meet the word most in three places: on a bag of fertiliser, in news about farming and food prices, and in history (people once made potash from wood ash to make soap and glass). It is a mass noun — you say 'some potash', not 'a potash' and not 'potashes'. It can also be used before another noun: a potash mine, potash fertiliser.