If a soil test reveals that your soil is lacking potassium, then you are probably looking for a way to supplement this important nutrient in your garden. Luckily, there are many options for high-potassium fertilizers.
So, what fertilizer is high in potassium? Fertilizers that are high in potassium include burned cucumber skins, sulfate of potash magnesia, Illite clay, kelp, wood ash, greensand, granite dust, sawdust, soybean meal, alfalfa, and bat guano.
Of course, you can use a mixture of any of these sources of potassium, depending on what you have available. You can also use fertilizers formulated as bloom or blossom boosters, which will have high potassium content.
Let’s get into more detail about just how much potassium each of these fertilizers contains, and how long it takes them to release into the soil.
What Fertilizer Is High In Potassium?
Here is a table with some of the best fertilizers with high potassium content. You can find more detail about each type of fertilizer after the table.
Note that N = Nitrogen, P = Phosphorus, and K = Potassium.
|Sulfate Of |
|0||0||3.5 to 8.3||NA|
|Kelp||1||0.5||4 to 13||slow|
|Wood Ash||0||5||3 to 7||fast|
|0||0||3 to 6||very |
|Sawdust||0.1||0.1||2 to 4||very |
|Bat Guano||5.5 to 8||4 to 8.6||1.5||medium|
|Manure||0.5 to 6.5||0.2 to 4||0 to 3||medium|
|Compost||1.5 to 3.5||0.5 to 1||1 to 2||slow|
Burned Cucumber Skins
Burned cucumber skins may not be the first thing you think of when you are looking for fertilizer for your plants.
However, burned cucumber skins contain 27% potassium by weight, meaning that it has the most potassium, pound-for-pound, out of any of the other fertilizers listed here.
Burned cucumber skins are a fast release fertilizer, and relatively easy to find. You can ask family or friends for skins leftover from peeling cucumbers for summer salads. You can even ask local restaurants that serve salads if they have cucumber skins available.
Burned cucumber skins also contain 11% phosphorus by weight, although they do not contain any nitrogen.
Sulfate of Potash Magnesia
Also known as potassium magnesium sulfate, this fertilizer contains 22% potassium by weight, making it a great potassium supplement – almost as good as burned cucumber skins.
Sulfate of Potash Magnesia is a specific type of potash, which means that it contains potassium in water-soluble form. It contains 11% magnesium by weight, but it does not contain any nitrogen or phosphorus.
For more information, check out this article on potash from Wikipedia.
Clay is a fertilizer that contains 3.5% to 8.3% potassium by weight. Potassium gets trapped within clay crystals, for example, in Illite. However, Illite contains no nitrogen or phosphorus.
For more information, check out this article on Illite from Wikipedia.
Kelp is a large seaweed that grows in “forests” underwater. Kelp has a brown color and grows in nutrient-rich water.
As a fertilizer, kelp contains 4% to 13% potassium by weight, making it a fairly good source of this nutrient. Kelp also contains 1% nitrogen and 0.5% phosphorus by weight.
Kelp has a slow release speed, and is effective for 4 to 6 months.
For more information, check out this article on kelp from Wikipedia.
Wood ash is simply what is left over after you burn wood. Nutrient content of ash will vary depending on the type of wood that was burned.
Generally, wood ash will contain 3% to 7% potassium by weight, making it another good source of this nutrient. Wood ash also contains 5% phosphorus by weight, although it does not contain any nitrogen (since nitrogen is burned off into the air when the wood combusts).
Wood ash has a fast release speed, and is effective for 1 to 4 months. It is alkaline (basic), so be careful about adding wood ash to soil with high pH.
Also, be careful about using ash from wood that was treated in any way, to avoid putting toxins into your garden.
For more information, check out my article on wood ash.
You can also check out this article on wood ash from Wikipedia.
Greensand is a type of sandstone, often rich in marine fossils, with a green color. It is also porous, meaning that it can retain both water and minerals.
As a fertilizer, greensand contains 5% potassium by weight. It also contains 1.5% phosphorus by weight, although it does not contain any nitrogen.
Greensand has a very slow release time, and is effective for 5 years or longer.
For more information, check out this article on greensand from Wikipedia.
Granite is an igneous rock that is usually white, pink, gray, or some mixture of these colors. Granite can be crushed into dust and used as a soil additive to supplement nutrients for your garden.
When used in this way, granite dust has 3% to 6% potassium by weight. It also contains small amounts of calcium, iron, magnesium, and manganese.
However, granite dust contains no nitrogen, and very little if any phosphorus.
Granite dust has a very slow release time, and is effective for 3 to 5 years or longer.
For more information, check out this article on granite from Wikipedia.
Sawdust is the leftover scrap dust produced by machines that cut wood, such as commercial sawmills or electric table saws in carpentry.
In addition to use in particle board or mulch, sawdust can be used as a fertilizer. When used as fertilizer, sawdust contains 2% to 4% potassium by weight.
Sawdust contains very small amounts of nitrogen and phosphorus by weight (less than 0.1%).
Sawdust has a very slow release time, and is effective for 2 to 4 years.
One caution about using sawdust is that it can cause nitrogen deficiency.
For more information, check out this article on sawdust from Wikipedia.
Soybean meal is used as a protein and energy source for people and animals. Often, soybean meal is produced by first extracting soybean oil from the beans. Soybean meal may also contain ground soybean husks.
Soybean meal can also be used as fertilizer, since it contains 2.4% potassium by weight.
Soybean meal also contains 6.5% nitrogen and 1.5% phosphorus by weight.
Soybean meal has a slow to medium release time.
For more information, check out this article on soybean meal from Wikipedia.
Alfalfa, or Lucerne, is a common cover crop in the legume family. Legumes contain specific bacteria in their roots which take nitrogen from the air, change it to a form that plants can use, and put it into the soil.
If you till alfalfa under the soil after growing it, your garden can benefit from the nutrients in the plant matter. Specifically, alfalfa contains 2% potassium by weight.
Alfalfa also contains 2.5% nitrogen and 0.5% phosphorus by weight. (Note: this does not even count the nitrogen that the bacteria in the alfalfa roots put back into the soil!)
Alfalfa has a slow release time, and is effective for 2 to 6 months.
For more information, check out this article on alfalfa on Wikipedia.
Guano is the excrement of seabirds and bats. Bat guano is a decent source of potassium, containing 1.5% potassium by weight.
It is difficult to find bat guano on your own, so you will probably need to buy it from a store or online.
Bat guano has a medium release time, and is effective for perhaps a month or two.
One caution is that bat guano has a low pH (very acidic), so it should be used with caution to avoid burning your plants with a sudden change in pH.
Only use bat guano directly in soils with high pH, or else mix it into your compost pile to dilute the acidity.
Here is a fun fact for you: bat guano that accumulates over many years can eventually provide a source of rock phosphate once the excrement hardens into layers of rock!
Bat guano also contains 5.5% to 8% nitrogen and 4% to 8.6% phosphorus by weight, making it a great all-around fertilizer that provides plenty of each nutrient.
For more information, check out this article on guano from Wikipedia.
There are many different types of manure, including cow, horse, pig, and chicken.
Cow and horse manure often contains only 0.5% to 2% potassium by weight.
Pig manure contains 0.4% to 1.2% potassium by weight.
Chicken manure contains 0.5% to 3% potassium by weight.
All of these manures have a medium release speed, and can be effective for two years.
Manures also contain small amounts of nitrogen and phosphorus as well, making them good all-around fertilizers. Just make sure to decompose manure completely before using it on your garden, to avoid burning your plants!
For more information, check out my article on manure.
Compost is made from kitchen scraps and yard waste, such as banana peels, orange rinds, grass clippings, and raked leaves.
Compost contains 1% to 2% potassium. It is a slow release fertilizer, and also contains nitrogen and phosphorus.
The best part about compost is that you can make your own in your backyard. For more information, check out my article on how to make compost.
Can You Have Too Much Potassium In Soil?
Yes, you can have too much potassium in your soil. Too much potassium in soil can prevent a plant from absorbing other important nutrients (such as nitrogen, calcium, and magnesium) from the soil.
The moral of the story is this: always get a soil test before adding any supplements to your soil. Make sure you really do have a potassium deficiency!
A soil test will also tell you if your soil is too acidic (low pH) or too basic (high pH), which can help you to decide which supplement to use.
For more information, check out my article on how to do a soil test.
By now, you have a much better idea of which fertilizers (both natural and man-made) have high potassium content by weight. You also know how to choose which one to use based on the time it takes for the fertilizers to release nutrients.
I hope you found this article helpful – if so, please share it with someone else who can use the information. If you have any questions about fertilizers that are high in potassium, please leave a comment below.