Potassium also known as the letter \"K\" as in the NPK content of fertilizers comes in a form called potash so a fertilizer with high potash is high in potassium. Please understand that some soil types such as sandy soil will be lacking in nutrients like potassium because they drain so quickly that the plants cannot absorb the potash when applied. This is why you should improve the soil with organic matter like compost that holds these nutrients so the plants can absorb it. This is also why a slow release plant food like Osmocote is more effective than spreading potash which may be in a highly soluble (as in it dissolves in water) form. There is also the danger of overfeeding your plants which you will see as brown tips on the leaves or even a yellow or scorched look to the foliage. Wood ashes is a natural form of potash, but like most soil amendments it works slowly to improve the soil over time. Animal bedding, manures and compost are the best ways to improve the structure and fertility of your soil for the best growth of vegetables. Keep adding organic matter to your soil each year and slowly it will improve so that you will need less fertilizer. Keep growing, Marianne Binetti