You can test your garden soil if you suspect a shortage of potassium. Often, however, you can tell what your plants are deficient in simply by looking at them. Physiological symptoms can give away the need for a bit of a potassium boost. Not all gardens will be deficient in potassium, though many can benefit from the addition of a little potassium, along with addition of the other vital elements of nutrition, nitrogen and phosphorus. Potassium is of particular benefit to flowering and fruiting plants.
Potassium can be added through the use of various soil amending additions. A compost that is rich in fruit and vegetable scraps will add potassium, though it is worth bearing in mind that the potassium in the mix is water soluble and can drain from a compost bin or heap in wet weather. For this reason, you may like to consider composting in place in growing areas, where plants are more likely to be able to take up the potassium before it is lost to them below their root zone. (You can add potassium rich scraps
directly to the soil – banana skins and citrus rinds are particularly rich in this nutrient.) Another thing that may help is to plant 'dynamic accumulators' such as dandelions and comfrey, with deep roots, which can be chopped and dropped to help return low-level nutrients beneath the surface to the topsoil, where it can be taken up by other plants.
One of the most traditional ways to add potassium, or potash, to your soil is to add wood ash from a bonfire or stove. Wood ash can be added in moderation though it is important to remember that wood ash will raise the pH of your soil, which can be a problem over time and is certainly not a good option for those who already battle with alkaline soil. If, however, you have an acid soil then wood ash could be a good solution to add potassium, either added directly beneath plants or added to your compost heap.