I found this useful hint on Barbara's blog, Postcards from Paris. Well, actually the whole blog is called something else but I have a link to the Postcards from Paris section in my side bar, so that has become my name for the whole blog....but now I see I should be calling it Paris Postcard! Oh dear, sorry Barbara if this is confusing!
Potassium is an element not that easy to come by in commercial organic fertilisers.
If you have a wood stove or fireplace, you can prepare an excellent potassium-rich liquid fertiliser with minimal effort.
Mix about 2 pounds of wood ash into a bucket containing 2.5 gallons of water.
Stir vigorously, leave to infuse for half an hour, then stir again. Leave the mixture to settle thoroughly.
Then skim off the floating debris and either decant the liquid from the solids at the bottom, or filter through a strainer or cloth.
This liquid will contain about 10 grams of potassium per quart (although actual concentrations can vary). It can be stored indefinitely in a plastic jug.
Use it undiluted at the rate of 1 quart per square yard to fertilise vegetables and flowers.