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: 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!)
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 that N = Nitrogen, P = Phosphorus, and K = Potassium. Also note that percentages are by weight – for example, 100 pounds of greensand contains 5 pounds of potassium (5/100 or 5% by weight).
|Fertilizer||% N||% P||% K|| Release|
|Sulfate Of |
|Kelp||1||0.5||4 to |
|Wood Ash||0||5||3 |
|Bat Guano||5.5 |
|4 to |
potassium content, along with their nitrogen content,
phosphorus content, and release speed.
Burned Cucumber Skins
Admittedly, burned cucumber skins are not the first thing you think of when you are looking for fertilizer for your plants. However, this strange item contains lots of potassium – probably more than you realize.
In fact, burned cucumber skins contain 27% potassium by weight. That means that it has the most potassium, pound-for-pound, out of any of the fertilizers listed 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.
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.
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. 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.
Potassium gets trapped within clay crystals (for example, in Illite). 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 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. Kelp also contains 1% nitrogen and 0.5% phosphorus by weight – not a balanced fertilizer, but nothing to laugh at either.
Kelp has a slow release speed, meaning that it releases nutrients slowly over a long time period as it decays. It 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. 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 5% phosphorus by weight, making it a good source of this nutrient as well. However, 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 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, 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. It is often rich in marine fossils and has a green color.
Greensand 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 it is effective for 5 years or longer.
For more information, check out this article on greensand from Wikipedia.
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% potassium by weight. It also contains small amounts 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 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 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 about using sawdust is that it can cause nitrogen deficiency. Your best bet might be 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 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, 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 it 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 (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 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 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 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 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.
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 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.
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.