Growing plants need nutrients to survive. Nitrogen, potassium and phosphate are the main three nutrients that gardeners add to their plants and farmers add to their crops. When you buy commercial fertiliser, the container will have three numbers on it representing per cent by weight of the chemical in question. The first number on the bag represents nitrogen, the second number phosphorus and the third potassium. Potassium carbonate, in the form of potash, is typically how gardeners add potassium to poor soil.
Burn firewood in a fireplace and collect the ashes.
Add the ashes to a large container of room temperature water and stir for several minutes. The soluble potassium and sodium salts will dissolve and the contaminants will settle to the bottom.
Pour the water into a pot, leaving the contaminants behind. Clean the container for later use. The water has a mixture of sodium and potassium chloride in solution.
Boil the water until you see a white material falling out of solution. Estimate how much water is left at this point and boil until half that amount of water remains in the pot.
Pour the hot solution into a container, leaving the solid precipitate behind. The liquid is still a mixture of sodium and potassium chloride. Allow the hot solution to cool to room temperature. Clean the pot for later use.
Pour the cooled solution off into another pot leaving the solid matter behind. Boil the pot to dryness and collect your potash left behind in the pot.
For an ongoing operation, you can use a fire pit to boil the solutions fired by wood and then restart the process with the wood ash.
Handle hot pots carefully.