Potassium is one of the key elements of fertilizer, listed as the "K" on the "N-P-K" rating of manufactured fertilizer. Farmers have been making their own potassium-rich fertilizers through the long history of agriculture, and so can any gardener. All a gardener needs is wood ash, although that wood ash must be collected with care, and for heavy applications the ash requires composting.
Collect wood ash from your fireplace, charcoal grill, brazier or anywhere else you are burning wood. Do not collect ashes from any fire that used fuel that might leave behind harsh chemicals, such as fire-starter logs and artificial charcoal briquettes.
Apply raw wood ash fertilizer by digging 3-inch-deep trenches around the plants with a hoe or garden trowel. Sprinkle a light, even layer of ash over the trench, and then turn the dirt over to refill the trench.
Create a potassium fertilizer for heavier applications by adding wood ash to your compost pile or tumbler. To make a potassium-rich fertilizer, every time you dump used coffee grinds, manure, egg shells and other waste products into the compost, shovel in an equal weight of wood ash. If you desire less potassium, use less wood ash.
Turn the compost pile weekly, and make sure the compost remains moist. Wait one year after you stop adding organic wastes and wood ash to the compost. The result is a potassium fertilizer.