"Potassium is a must-have nutrient for growing both vegetables and fruits."
https://homegardenfirst.com/organic-sources-of-potassium-for-plants-organic-potassium-sources/
"potassium is supplied to agricultural land in a mineral form and it dissolves in the soil water and is available to the plant.
Typical sources include mined rock powders and wood ash. Of course, manures, compost and other organic materials are potassium sources, too, because even though the concentration of potassium in them is pretty low, typically a lot of material is applied to a field.
Before considering potassium sources, determine whether you need potassium – which is not as simple as applying the amount of potassium a crop needs."
https://www.mofga.org/resources/soil/potassium/
"There are many excellent K sources allowed for organic crop production, including soluble minerals such as langbeinite, sylvinite, and potassium sulfate. Potassium sources such as wood ash, greensand, and seaweed can also supply K but require special management because of their low nutrient content, their effect on soil pH, low solubility, or bulky nature. The concentration of K in manures and composts is highly variable, but it is generally quite soluble and available for plant uptake. Some rock minerals may supply a portion of the K requirement of plants, but many are too insoluble to be of practical significance."
https://journals.ashs.org/horttech/view/journals/horttech/17/4/article-p455.xml
"There are several organic potash sources that can provide potassium in organic vegetable gardens.
Greensand, kelp meal, and hardwood ashes are all good organic potassium sources. All are readily available."
https://www.grow-it-organically.com/organic-potash-sources.html
"Compost is full of nutrients, including potassium, especially if it is beefed up with banana peels and other fruit and vegetable waste. The potassium compounds in compost are water-soluble, which makes them readily available to plants but also likely to leach out of your compost pile over time."
https://todayshomeowner.com/lawn-garden/guides/organic-sources-of-potassium-for-your-lawn-or-garden/
"Langbeinite is a mineral that is made of important nutrients for plants: potassium, magnesium, and sulfur. It’s found only in a few places. In the U.S., langbeinite is extracted from mines close to Carlsbad, New Mexico. The evaporation of ancient oceans left behind unique minerals, including this one."
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/langbeinite-fertilizer-in-gardens.htm
"Certain types of K fertilizers are approved for use in organic farming. If potassium is needed there are several commercial K fertilizers, such as langbeinite or potassium sulfate, which may sometimes be used with certain restrictions in organic farming."
https://njaes.rutgers.edu/e364/