Potassium fertilizer is the “K” in the N-P-K numbers listed on every bag of fertilizer. All fertilizers have three numbers on the bag, such as 12-7-0 or 10-10-10. The numbers represent the relative amounts of Nitrogen (N), Phosphorus (P), and Potassium (K) in the fertilizer. They are always listed in that order, and are referred to as that fertilizer’s N-P-K value.
Soils vary in the amount of naturally-occurring potassium that is available to plants. The soils in Colorado where I live generally have abundant potassium, and so we really don’t need to amend it, but a soil test will show you whether or not your soil may be deficient.
There are a number of home soil test kits on the market, but I prefer to send a soil sample to the local Extension Service, because I know they are accurate, they are relatively inexpensive, and they also include recommendations for fertilizer application rates, if needed. All states in the US have a Cooperative Extension Service at their land-grant college that will provide this service for about $18-20.
Why Plants Need Potassium
Potassium is one of the most critical minerals needed by plants for a variety of physiological functions. Plants need it for:
Signs of potassium deficiency in plants are leaf edges turning brown (looking as though they are getting scorched) and leaves turning yellow, especially at the tips, while the veins remain green.
Inorganic Sources of Potassium
The potassium fertilizer used in conventional, non-organic commercial agriculture is synthetically manufactured and is considered “inorganic” fertilizer. Common forms, listed with their N-P-K values, are:
Muriate of potash (potassium chloride) (15-15-15). While this a naturally-occurring, mined substance, it kills beneficial bacteria in the soil, so is not recommended in the organic garden
Unlike the production of nitrogen and phosphorus fertilizers, the manufacture of potassium fertilizer appears to be relatively harmless to the environment, not being a major pollution source.
Organic Potassium Sources
Potassium occurs naturally in both plant and mineral forms. In the organic garden it is better to use these sources of potassium, which are slower releasing and do not harm soil microorganisms. They vary in their N-P-K numbers depending on brand, so the numbers listed below are averages.