Potassium is one of the Big Three nutrients that plants need in large amounts (the other two are nitrogen and phosphorous). These are the essential nutrients that are represented by three numbers, such as 5-10-15, on bags of chemical fertilizers.
If you’ve had the soil of your organic garden tested and learned that it is very low in potassium, you may be looking for ways to boost the amount of this essential plant nutrient in your organic garden.
The good news? There are several good sources for potassium available for organic gardens.
Greensand – This is a mined product, made from natural deposits of a marine sediment called glauconite. It is smashed into very small bits (making it into “sand”) for you to mix into your soil. The potassium content can vary between brands. You will need to read the label of your bag to know how much potassium (the percentage) is in there.
Greensand does not contain any nitrogen; some brands may contain a tiny bit of phosphorus, but not enough to really count if your garden needs phosphorus, too. The potassium is released SLOWLY, so if your garden’s need is dire, you might want to use a faster-release potassium source along with the greensand.
When I make my own organic potting mix, I add a little greensand (about a half cup for a 6-gallon batch of potting mix). McGee and Stuckey, in their book The Bountiful Container, also recommend adding a bit of greensand to organic potting mixes.
Wood Ashes – This is a fast-release and FREE source of potassium for home gardens. Wood ashes may be as much as 3-7% potassium! This is similar to the percentage in many bags of greensand (my current bag has less), making wood ashes an option to consider for gardens that need a lot of potassium Right Now. The potential drawback to this organic potassium source is that it will raise the pH of your garden’s soil.
Did you have the soil from your organic garden tested this year? If you did, you will have a record of the soil pH, which is how acidic or basic it is. A pH number below 7 is acidic. A pH number higher than 7 is basic. Most garden vegetable plants will be more productive when grown in soil that is in the 6-7 pH range (slightly acidic). If your soil pH is already in that desired range — or higher — then wood ashes are not a good potassium choice for your garden.
Sul-Po-Mag – This is another mined source of potassium, available to gardeners as bags of pulverized rock. The name Sul-Po-Mag is shorthand for the longer name of Sulfate of Potash Magnesia. This has a very high percentage of potassium, as much as 22%. Even though it is from rocks, it is considered a fast-release source. If your organic garden needs a lot of potassium right away, and the pH is high enough that wood ashes are not a good choice, then this may be what you want. Sul-Po-Mag also provides sulfur, magnesium, and some micronutrients. This product is generally pH-neutral, meaning that it will not swing your soil’s pH in either direction.
Locally, we have used Sul-Po-Mag when mixing up batches of organic fertilizer for community gardens that have a soil-pH higher than 7.
Kelp Meal – This is a slower-release source of potassium. It is not a concentrated source (about 1% potassium), but it also provides nitrogen, phosphorus, and a host of essential micronutrients that can improve the nutrient state of poor soils. If you are working with native soil, in an in-ground garden in the Southern US, and your organic garden is fairly new, you may want to choose kelp meal as your potassium source. In my area, the clay soils are all worn-out (decades of cotton fields) or washed-out (erosion from high rainfall), and benefit greatly from a dose of kelp meal.
How much of any of these should you use? If you have a report from a soil lab, the recommendation it offers for a chemical fertilizer can be converted to organic amendments, if you are reasonably good at math. The linked UGA publication includes the equations to use. I have done these conversions for our local community gardens.
An alternative is that you mix up a batch of organic “10-10-10” (or another listed formula), following the recipe in the above-linked UGA publication about converting to organic amendments. The recipes on the last page of the document list options for the nitrogen source (pick one from the list), options for the phosphorus source (pick one), and options for the potassium source (pick one).