Certain weeds indicate a soil rich in potassium. These are march mallow, knapweed, wormwood, opium poppy, fumitory, russian thistle, tansy and sunflower.  Red clover, however, is a good indicator of potassium deficiency.  It will disappear with increasing acidity.

Tobacco takes up potassium in its leaves and stalks and, as a result, is  a good plant for composting provided it has not been sprayed with synthetic chemicals. Vegetables that like potassium are celeriac and leek.