Of all the “Big 3” soil ingredients, the least understood is definitely potassium. While the effect of potassium on plants is fairly clear – we can see that it improves how well a plant grows and produces – exactly why and how it works is not as well-known.
Sadly, as a gardener, you don’t really need to know the why and how in order to be hurt by a potassium deficiency in plants. But luckily, you also don’t need to know the how and why in order to fix that deficiency either. Let’s take a look at how potassium affects the plants in your garden and how to correct a potassium deficiency.
- 1 What causes potassium deficiency in soil?
- 2 What does potassium do for plants?
- 3 How much potassium do I add to my soil?
- 4 Can too much potassium be bad for my garden?
- 5 What is the best source of potassium for plants?
- 6 What fertilizer is high in potassium?
- 7 What do I have at home to add potassium to plants?
- 8 What’s the best way to increase potassium in soil?
What causes potassium deficiency in soil?
While nitrogen and phosphorus are found in organic matter, potassium is typically not. Soil organisms have a much lower requirement for potassium than plants do. This means that most of the potassium is released quickly when organic residues decompose and it can be more likely to leach out of the soil.
How potassium will react in the soil is reliant on a couple of things: first of all, the potassium ion is small and can be trapped inside crevices within clay particles. This leaves them trapped and unavailable for use in the soil. The other factor that influences potassium levels in the soil is more complicated. It involves a soil mechanism called cation exchange.
This exchange is caused when small clay and humus particles develop a negative electrical charge. The negatively charged particles then attract positively charged ions, also known as cations, which include potassium. As long as the soil is free of sand, high levels of potassium typically exist.
While some plants, either because they’re grown in an acidic environment or where there are lots of helpful soil bacteria, are able to extract potassium directly from rock powders, most plants cannot – leading to potassium deficiencies in your crops.
And by the time crop nutrient deficiencies become visible, yield damage has already occurred to some degree. Deficiency symptoms occur first on older plant tissue, beginning with yellowing at the leaf margins, and then followed by that plant tissue dying.
Potassium deficiencies can develop in plants even when soil tests show a sufficient soil pH because of the cation ratios. Potassium that is applied by fertilizer or potassium released from a fixed state in the soil cannot displace other ions bound tightly together by cation exchange. So it travels deeper into the soil profile until it finds a space.
While low soil pH can cause potassium deficiencies, the symptoms may not appear on your crops until the pH level is well below 5.0. In soil that are lime-heavy, calcium can displace potassium from the cation exchange, which means that it can leach down through the soil profile to where it is no longer available for your plants.
The amount of oxygen in the soil can also affect its potassium uptake. When soil oxygen becomes low – which can happen in highly compacted or over-watered soil, root respiration is decreased, which can limit the uptake of potassium in the soil.
What does potassium do for plants?
Called one of the “big 3” nutrients, it’s clear that potassium is important to plant growth and development.
Though it’s not a structural part of plant tissue to any significant degree, potassium is associated with the movement of water, nutrients and carbohydrates in plant tissue. It helps regulate the opening and closing of the stomata, which regulates the exchange of water vapor, oxygen and carbon dioxide within the plant.
Potassium also plays a role in activating the plant’s enzymes, which can affect the production of protein, starch and adenosine triphosphate (ATP). The production of ATP can regulate the rate of photosynthesis.
If potassium is deficient in the garden soil, and it’s not supplied in adequate amounts, it stunts plant growth and reduces yield. Among other things, potassium helps plants:
- Grow faster and stronger
- Use water better, increase root growth and resist drought
- Fight off disease and pest infestations
- Produce more crops
How much potassium do I add to my soil?
Unless you know the history of your soil well enough to know that it’s potassium-deficient, it would be wise to test the nutrient content before adding potassium.
If the soil is proven to be low in potassium, then you can apply about 50-100 lbs of commercial potash per acre of soil. If you plan to add nitrogen at the same time, then you should apply them both in equal amounts.
If your soil is also low in magnesium, and you plant to add potassium only, it’s best to spread the fertilizer more sparsely but more frequently.
Can too much potassium be bad for my garden?
Though a little extra potassium generally isn’t cause for worry, very potassium-rich soil can be a problem. As important as it is to their overall health, too much potassium can be dangerous for plants because it affects the way the soil absorbs other critical nutrients.
Lowering soil potassium may also prevent excess phosphorus in the soil from running into the local waterways – where it can increase the growth of algae which could eventually kill aquatic organisms.
If you do need to reduce potassium levels in your soil, you should choose fertilizers with a low number or a zero in the K position – or you could skip the fertilizer entirely. Many plants can still thrive without it. An alternative to chemical fertilizers, organic fertilizers such as manure generally have lower N-P-K ratios. What’s more, the nutrients in manure break down slowly, which may prevent potassium buildup.
You may also choose to sift the soil and remove as many rocks as possible. Some rocks – like feldspar or mica – can release potassium into the soil. Loosening potassium-rich soil with a garden fork or shovel, then watering deeply will also dissolve and flush out the surplus potassium. You should allow the soil to dry completely, then repeat the process two or three more times.
What is the best source of potassium for plants?
While experts don’t always agree on the best source of potassium for plants, we know that there are several very good options. Some commercial fertilizers are available in synthetic or organic formulations, and there are even ways to add potassium to your garden using household items.
Wood ash can supply a quick hit of potassium and should be applied shortly before the crop requires it. But because it is also very alkaline, you may wish to avoid using it unless you also need to raise the soil pH level.
Potassium sulfate typically contains a formula of 50% potassium – it is available in both natural and synthetic forms. It’s an expensive option for adding potassium to your soil as it doesn’t contain significant amounts of any other nutrient. It is, however, highly soluble. So it’s best to apply potassium sulfate just before planting or while your plants are growing – otherwise a considerable amount of the nutrient will be leached from the soil.
The most common source of potassium in conventional commercial fertilizers is Sul-Po-Mag (which typically contains 20% potassium and 11% magnesium per volume). This mined rock is also a very economical choice for gardens, and some formulations are entirely organic while other may have chemical additives. Sul-Po-Mag is a great choice if your garden beds also need magnesium and you don’t need to increase their pH level. It also provides immediate nutrition so use it while your plants are growing.
You could also use a potassium-chloride-based fertilizer, or one containing potassium nitrate if these secondary nutrients are also lacking in your soil. Whatever you choose, remember that potassium is very soluble, so you should always add potassium fertilizers close to when crops will be growing. If you add them to your garden in the fall while preparing the beds for spring crops, most of the potassium will have leached out of the soil by then.
Soil microbes typically hold both nitrogen and phosphorus in their bodies in the soil, but they have little use for potassium. So if you plow down a cover crop that is high in potassium, you should then plant a cash crop or another cover crop to grab it up, because the soil microorganisms won’t store it.
What fertilizer is high in potassium?
There are two main types of potassium fertilizers in which the potassium will be combined with either chloride or sulfate. Potassium chloride (KCl) is available in three different grades, containing either 50%, 41%, 33% potassium per volume. Both the 33% and 41% formulations usually contain substantial amounts of sodium chloride (NaCl) – also known as table salt – which are fantastic choices for salt-loving crops.
Potassium sulfate is usually manufactured by creating a chemical reaction in combining the chloride with sulfuric acid. This fertilizer typically contains about 43% potassium per volume.
Sulfate potash magnesia (or Sul-Po-Mag) is essentially a mixture of potassium sulfate and kieserite containing about 20% potassium and 11% magnesium per volume. This is a useful fertilizer to apply to when your soil also needs magnesium.
Potassium nitrate fertilizers are mainly used under glass for intensive horticultural applications.
Though wood ash is a great source of potassium, it does have certain drawbacks: ashes can be caustic, and could cause the soil pH to rise significantly; it’s also quite difficult to obtain enough wood ash to treat larger areas.
Rock powders such as granite dust or greensand can also add a significant amount of potassium to the soil. They’re a good long-terms solution as spreading 3-5 tons of rock powder per acre can boost the potassium level for up to 3 or 4 years, and potentially even longer if other sources of potassium are also used.
Granite dust has a fairly neutral pH, but greensand is acidic, with pH levels between 1.0 and 3.5 but it can be neutralized by adding lime. Though the soil may be temporarily disturbed locally by the acidity of greensand, the long-term effect should be negligible.
Though they’re higher-priced than some other options, potassium rock powders do have 3 advantages over soluble fertilizers:
- They mimic the natural tendency of soil minerals to release their potassium slowly, eliminating over-consumption by the plants if no other significant source of potassium is present;
- they can contain trace elements of other minerals and nutrients;
- and require less energy to produce, though this saving is partially offset by the greater amount of energy required for transportation.
What do I have at home to add potassium to plants?
You don’t have to invest in expensive chemical additives in order to increase the potassium level of your soil. There are many potassium-rich treatments using home-made alternatives.
You may wish to amend your garden with home-made liquid comfrey feed, or by adding seaweed meal, composted bracken, or compost rich in decayed banana peels. By adding fruit to your compost, it’s easy to boost the potassium content – you simply need to cut your banana peels into small pieces then mix them into your compost pile.
You can also create a liquid version by placing some of the banana peel pieces in a spray bottle filled with warm water. After allowing the peels to ferment in the water for two weeks, just spray the liquid onto the soil.
If you burn wood on your property, you can use the wood ash as fertilizer. It has a high natural potassium content but be cautious because it can alter the pH level of your soil. Gather the potassium-rich ashes once your fire is out and add them to soil as you create a compost pile, sprinkling a small amount on each layer. Allow each layer of the ashes to dissolve before advancing to the next layer.
Coffee grounds also contain significant amounts of potassium – up to 12 grams per kilogram of dry coffee grounds. And while you can simply spread the grounds on the soil surface, they’ll be more effective if you dig them in to about 6”-8” below the surface.
Another easy solution for increasing the amount of potassium available to your plants is to ensure the soil is kept moist. Low levels of soil water can impact the uptake of potassium by plant roots. If your soil is acidic, you may wish to add lime – this can increase potassium retention in some soils by reducing leaching.
What’s the best way to increase potassium in soil?
While each of these options can add significant amounts of potassium to the soil, there isn’t one right way to amend your soil. You should always consider the specific needs of your plants, as well as the existing soil conditions, and other climate conditions when determining which fertilizer is best in any particular case.
So what does your garden need?