If a soil test reveals that your soil lacks potassium, you will need to find a way to add this important nutrient to your garden. Luckily, there are many options available for high-potassium fertilizers.
So, what fertilizer is high in potassium?Fertilizers that are high in potassium include: potassium chloride, potassium sulfate, burned cucumber skins, sulfate of potash magnesia, Illite clay, kelp, wood ash, greensand, granite dust, sawdust, soybean meal, alfalfa, and bat guano. Some of these fertilizers also contain nitrogen, phosphorus, and other important nutrients for plants.
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 booster (or blossom boosters), which will also have high potassium content.
In this article, we’ll talk about some fertilizers that are high in potassium and exactly how much each one contains. You’ll also get an idea of how long it takes these fertilizers to release their nutrients into the soil (release speed).
Let’s get started.
(You can also watch my YouTube video on this topic if you prefer, or you can see it below.)
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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 in the article after the table.
Note the numbers given in the table are percentages by weight, and that N = Nitrogen, P = Phosphorus, and K = Potassium. For example, 100 pounds of greensand contains 5 pounds of potassium (5/100 or 5% by weight).
| Potassium Source | % N | % P | % K | Speed |
|---|---|---|---|---|
| Potassium Chloride (Muriate of Potash) | 0 | 0 | 60 to 62 | fast |
| Potassium Sulfate | 0 | 0 | 50 | varies |
| Burned Cucumber Skins | 0 | 11 | 27 | fast |
| Sulfate Of Potash Magnesia (Sul-Po-Mag) | 0 | 0 | 22 | NA |
| Clay (Illite) | 0 | 0 | 3.5 to 8.3 | slow |
| Kelp | 1 | 0.5 | 4 to 13 | slow |
| Wood Ash | 0 | 5 | 3 to 7 | fast |
| Greensand | 0 | 1.5 | 5 to 7 | very slow |
| Granite Dust | 0 | 0 | 3 to 6 | very slow |
| Sawdust | 0.1 | 0.1 | 2 to 4 | very slow |
| Soybean Meal | 6.5 | 1.5 | 2.4 | slow to med |
| Alfalfa (Lucerne) | 2.5 | 0.5 | 2 | slow |
| Bat Guano | 5.5 to 8 | 4 to 8.6 | 1.5 | med to fast |
| Manure | 0.5 to 6.5 | 0.2 to 4 | 0 to 3 | med to fast |
| Compost | 1.5 to 3.5 | 0.5 to 1 | 1 to 2 | slow |
For more information, check out this article on NPK values of fertilizers from the Oregon State University Extension.
Potassium Chloride (Muriate Of Potash)
Potassium chloride is a heavy hitter when it comes to potassium, offering around 60% potassium by weight. Also called muriate of potash, this highly-water soluble fertilizer is often used in farming to treat potassium-deficient soil.
Depending on how it is processed, potassium chloride may be white or red (due to iron).
Potassium chloride does not contain any nitrogen or phosphorus, nor any other nutrients (aside from chlorine). So, you will have to look elsewhere to supplement the other important plant nutrients.
One problem with potassium chloride is the high salt/chlorine content. If you apply it directly to plants, you will burn them (or their roots).
You are better off using a formulation that is watered down a bit to decrease the chance of burning plants. You can also “side-dress” plants with this fertilizer (this means putting it near the plants, not directly on top of them).
Another option is to separate applications of this fertilizer into multiple sessions.
Of course, if you are worried about damaging your plants, you might want to avoid potassium chloride and go for something a little less salty.
Potassium Sulfate
Potassium sulfate is another strong option when it comes to potassium, offering around 50% potassium by weight. This fertilizer is water soluble – but only about 1/3 as soluble as potassium chloride.
Potassium sulfate is often used in farming to treat potassium-deficient soil for crops that are sensitive to high salt or chlorine concentrations.
Potassium sulfate also contains some sulfur, which is another important plant nutrient. Just remember that this fertilizer will acidify your soil somewhat, depending on the amount you apply.
Potassium sulfate does not contain any nitrogen or phosphorus. So, you will have to look elsewhere to supplement the other important plant nutrients.
If you want to space out doses of potassium for plants, you can use separate applications of this fertilizer into multiple sessions.
One drawback of potassium sulfate is that it is more expensive than potassium chloride.
Burned Cucumber Skins
Admittedly, burned cucumber skins are not the first thing you think of when you are looking for fertilizer for your plants. They are also difficult to get a hold of without making them yourself!
However, this strange item is an organic source of potassium for plants. In fact, it contains lots of potassium – probably more than you realize.
Burned cucumber skins contain 27% potassium by weight. That means that it has the 3rd highest potassium percentage out of the 15 listed fertilizers here.
Burned cucumber skins are a fast release fertilizer, meaning that the nutrients become available soon after they decompose. They are also relatively easy to find.
You can ask family or friends for cucumber 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. However, they do not contain any nitrogen – the one downfall of an otherwise solid fertilizer.
If you don’t like cucumber skins but don’t want to burn them, you can always add them to your compost pile to give your plants an extra boost.
Sulfate Of Potash Magnesia
Also known as potassium magnesium sulfate (or Sul-Po-Mag), this fertilizer contains 22% potassium by weight. With such a high potassium content, Sul-Po-Mag makes a great 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 also contains 11% magnesium by weight, which is helpful if your plants are deficient and you don’t have any Epsom salt (or don’t want to use it).
Remember that Sul-Po-Mag does not contain any nitrogen or phosphorus. As such, it is not a balanced fertilizer. However, it does have its uses.
According to the University of Minnesota Extension, potassium magnesium sulfate makes a good starter fertilizer for corn. It is also a good way to provide magnesium without changing your soil pH.
For more information, check out this article on potash from Wikipedia.
Clay (Illite)
Clay is a fertilizer that contains 3.5% to 8.3% potassium by weight. It cannot compare to burned cucumber skins or Sul-Po-Mag, but it is still a solid choice if you need to add some potassium to the soil.
According to the University of Maryland, potassium gets trapped within clay crystals (for example, in Illite).However, the clay is unavailable at first, making it a slow-release potassium fertilizer.
Clay is a good option if you want to maintain an organic garden that does not use artificially created chemicals.
Just remember that Illite contains no nitrogen or phosphorus, meaning that it is not a balanced fertilizer.
For more information, check out this article on Illite from Wikipedia.
Kelp
Kelp is a large seaweed that grows underwater in large groups (these groups are sometimes called “forests”). 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 good source of this nutrient. The exact percentages of potassium will depend on the form (powder, liquid, or meal).
Kelp also contains 1% nitrogen and 0.5% phosphorus by weight.
Kelp has a slow release speed, meaning that it releases nutrients slowly over a long time period as it decays. Kelp meal is effective for 4 to 6 months (it will release nutrients faster in powder or liquid form).
For more information, check out this article on kelp from Wikipedia.
Wood Ash
Wood ash is simply what is left over after you burn wood. The nutrient content of wood ash will vary, depending on the type of wood that was burned.
Generally, wood ash will contain 3% to 7% potassium by weight. This makes it another good natural source of this nutrient.
Wood ash also contains some phosphorus, magnesium, and sulfur, making it a good source of more than one plant nutrient. However, wood ash 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 it is effective for 1 to 4 months.It is alkaline (basic), so be careful about adding wood ash to soil that already has a high pH (learn how to do a soil test to find out for sure).
Also, be careful about using ash from wood that was treated with chemicals. That way, you can avoid putting toxins into your garden.
For more information on using wood ash in the garden, check out my article here.
You can also check out this article on wood ash from Wikipedia.
Greensand
Greensand is a type of sandstone. It is often rich in marine fossils and has a green color. (Glauconite is a large part of what makes up greensand).
Greensand is also porous, meaning that it can retain both water and minerals.
As a fertilizer, greensand contains 5% to 7% 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 it is effective for 5 years or longer.
For more information, check out this article on greensand from Wikipedia.
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Granite Dust
Granite is an igneous rock. It 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% potassiumby weight. It also contains smallamounts of calcium, iron, magnesium, and manganese.
The downside is that granite dust contains no nitrogen, and very little phosphorus.
Granite dust has a very slow release time, and it is effective for 3 to 5 years or longer.
For more information, check out this article on granite from Wikipedia.
Sawdust
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 canbe used as a fertilizer. When used asfertilizer, sawdust contains 2% to 4% potassium by weight.
Sawdust also contains very small amounts of nitrogen and phosphorus by weight (less than 0.1%).
Sawdust has a very slow release time, and it is effective for 2 to 4 years.
One caution: using sawdust can cause nitrogen deficiency in soil. Your best bet is to compost the sawdust first (you can learn more about how to compost sawdust in my article here).
For more information, check out this article on sawdust from Wikipedia.
Soybean Meal
Soybean meal is used as a protein and energy source for both 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. This makes it a good all-around fertilizer for your garden.
Soybean meal has a slow to medium release time.
For more information, check out this article on soybean meal from Wikipedia.
Alfalfa (Lucerne)
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, yourgarden can benefit from the nutrients in the plant matter. Specifically, alfalfa contains 2% potassiumby 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 it is effective for 2 to 6 months.
For more information, check out this article on alfalfa on Wikipedia.
Bat Guano
Guano is the excrement of seabirds and bats. Bat guano is a decentsource of potassium, containing 1.5% potassium by weight.
It is difficult to find bat guano on your own (unless you often embark on adventures like Indiana Jones). So, you will probably need to buy it from a store or online.
Bat guano has a medium release time, and it is effective for perhaps a month or two.
One caution is that bat guano has a low pH (that is, it is very acidic). So, use it 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 yourcompost 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 byweight, making it a great all-around fertilizer that provides plenty of eachnutrient.
For more information, check out this article on guano from Wikipedia.
Manure
There are many different types of manure, including cow, horse, pig, andchicken.
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 they 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 decomposemanure completely before using it on your garden, to avoid burning your plants!
For more information, check out my article on manure.
Compost
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 it also contains nitrogen and phosphorus.
The best part about compost is that you can make your own, right in your backyard. For more information, check out my article on how to make compost.
What Is The Most Common Potassium Fertilizer?
Potassium chloride is the most common potassium fertilizer, followed by potassium sulfate. Potassium chloride is probably more popular due to its lower cost, even though there is more risk of salt damage to plants.
According to the University of Minnesota Extension, some of the most common potassium fertilizers are:
- Potassium Chloride (highest potassium content, at 60% to 62%) – according to the University of Mississippi Extension, this fertilizer accounts for over 90% of the K sold in the U.S. and Canada.
- Potassium Sulfate (also provides sulfur to plants) accounts for about 6% of agricultural sales of K.
- Potassium-Magnesium Sulfate (also provides sulfur and magnesium)
- Potassium thiosulfate (also provides sulfur)
- Potassium Nitrate (also provides nitrogen)
What Is The Fastest Way To Add Potassium To Soil?
The fastest way to add potassium to soil is to use potassium chloride. However, this adds a large amount of potassium all at once – possibly too much, unless you water it down or spread out the application over multiple sessions.
Potassium chloride also adds salt/chlorine to soil, so it should not be used for plants that are sensitive to salt or chlorine (such as potatoes, tomatoes, cucumbers, strawberries, and others).
Is There A Liquid Potassium Fertilizer?
There are various formulations of liquid potassium fertilizer if you wish to go that route, including:
Can You Have Too Much Potassium In Soil?
It is possible to 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 before using additives that are high in potassium!
A soil test will also tell you if your soil is too acidic (low pH) or toobasic (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.
Does Epsom Salt Have Potassium?
Epsom salt does not have potassium. Epsom salt is magnesium sulfate, so it only contains magnesium and sulfur (and some oxygen atoms as well!) So, Epsom salt is not a source of potassium for plants.
Conclusion
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 the right fertilizer for your garden, based on the time it takes to release nutrients.
If you want some ideas for how to add nutrients to your garden soil naturally, check out my article here.
I hope you found this article helpful – if so, please share it with someone else who can use the information.
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~Jonathon
