Boosting Your Yields with Potassium Supplements

By Harley Smith
Published: February 27, 2014 | Last updated: June 13, 2022 10:25:28
Key Takeaways

To keep the quality of your fruits and flowers high, potassium supplements can make a big difference.

Want to keep your garden growing strong during the fruiting and flowering process? Giving your plants the right amount of potassium is key. During heavy fruiting and flowering, plants can deplete the potassium in the root zone quickly, sometimes in as little as three or four days.


Potassium (K) is a catalyst for carbohydrate metabolism, so if your plants don’t get the amount they need, sugar production slows down and your plants have a hard time storing up the energy they need for fruit and flower development. Plant growth stalls, and the quality of fruits and flowers declines. To keep the quality of your fruits and flowers high, potassium supplements can make a big difference.

Potassium Supplement Products

There are a lot of phosphorus and potassium (bloom booster) products on the market that provide extra phosphorus and potassium during the fruiting and flowering stage. Phosphorus is especially beneficial at the beginning of the flowering stage since it provides energy for earlier flowering and the production of more flowering sites.


Potassium, on the other hand, is best during heavy fruiting and flowering. For example, if tomato plants begin to develop a potassium deficiency, the fruit will become watery with low sugar content and poor shelf life.

That’s why commercial hydroponic tomato growers often use potassium sulfate as a boost to their nutrient formula during the fruiting and flowering stage. Maintaining high potassium-to-nitrate ratios promotes fruit and flower quality, with higher sugar content, better shelf life and improved flavors.

If you use an organic bloom stimulant, the need for potassium supplements is even more important. In 2007, I had the privilege of visiting a prestigious research greenhouse in Belgium where they were performing scientific trials on organic bloom stimulants. They were growing sweet peppers in the trial, with and without bio-stimulants.


Even the control plants were impressive. The untreated sweet pepper plants were 12 foot tall and loaded with peppers! The treated plants in the next row were even more productive.

Halfway through the trial, the plants treated with organic bloom stimulants had already achieved a 10 per cent increase in yield, and the numbers were continuing to rise. Not only were there more fruit and flowers, but the fruit was heavier and more fully developed.


There was a catch, however. As the pepper plants were putting on more weight with fruits and flowers, the potassium was being pulled out of the nutrient solution quickly. Every week, the scientists took a nutrient solution sample for testing and charted the amount of every mineral in the nutrient solution.

As production increased, potassium levels decreased, while most of the other minerals remained relatively unchanged. Unfortunately, the scientists weren’t allowed to add extra potassium to the experiment—if they had added more potassium to the nutrient solution, it would have been impossible to know for sure if the increased yield was a result of the bio-stimulants or the extra potassium.

So to keep the results clear, only the bio-stimulants were added in the experiment, even though potassium was sure to become the limiting factor.

A 10 per cent increase in yield is significant, but I’ve often wondered how much more the plants could have produced if the researchers combined bloom stimulants with just the right amount of supplemental potassium.

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What Nutrient Supplements to Add

Without question, if you want to push your plants to reach their true genetic potential, always add potassium supplements to your feeding schedule during the heavy fruiting and flowering period. But how do you know which potassium supplements work the best?

Practically any off-the-shelf P-K boost formula will benefit plants during general flowering, but for best results, a targeted potassium supplement may be a better choice to help your plants reach their peak fruit and flower production.

Near the end of flowering, plants don’t need additional nitrogen or phosphorus in the feeding schedule. Instead of continuing to give your plants supplements derived from potassium nitrate or mono-potassium phosphate, try a potassium supplement derived from potassium sulfate, which provides the potassium boost plants need without any additional nitrates or phosphates.

The sulfur in potassium sulfate is an added bonus, since sulfur compounds help “turn on” flowering genes in the plant and contributes to flavors and aromas.

Just remember, “Too much of a good thing isn’t a good thing!” A lot of gardeners tend to think that if a teaspoon of fertilizer is good, two teaspoons is twice as good. Not so. In plant nutrition, it’s all about balance. Ideally, we should give the plant only what it needs, when it needs it.

Although it’s true that plants need an extra dose of potassium during the peak of their generative phase, they don’t need an overdose. The goal is to replace the potassium the plant used up—if you overdo it and give your plants more potassium than they need, it could cause a deficiency in other nutrients, especially calcium and magnesium.

Remember, don’t just arbitrarily pile on every additive that claims to improve flowering. Using too many additives is one of the most common causes of nutrient imbalances in the garden. Moderation is the key.

How Much Potassium is Too Much Potassium?

So how do you know how much is too much? Your plants will tell you. For example, potassium toxicity commonly shows up as a magnesium deficiency, as potassium and magnesium ions can compete with one another for uptake by the plant.

Magnesium deficiency shows up in the older growth near the bottom of the plant as interveinal chlorosis. In other words, the veins of the lower leaves stay green, but the tissue between the veins will start to turn yellow.

If you start to see symptoms of magnesium deficiency, back off on your potassium supplements a little. And don’t worry; correcting a magnesium deficiency is easy. Simply add a little calcium and magnesium supplement (cal-mag) to your nutrient solution or spray a mild solution of magnesium sulfate on the bottom leaves of your plants. The leaves will green up in a matter of days.

The point of this story is: don’t be afraid of using a generous dose of potassium supplements during flowering, just use them as directed for best results.

It’s always best to use the purest, most water-soluble forms of potassium you can find. Generally speaking, try to stay away from agricultural-grade potassium supplements if you can. They often use a chemical extraction process and may have unacceptable levels of impurities.

The best natural forms of potassium sulfate supplements are “soluble fines”, and some even qualify for OMRI certification for organic gardening. Don’t be afraid to ask questions before you buy. Educated sales staff should know the source of the fertilizer salts they sell, or they should be able to quickly find out from the manufacturer.

The bottom line is that potassium is the health element. If you want healthier plants with higher-quality flowers and sweeter fruit, potassium is the key. As the summer progresses and your plants begin to fill in with fruits and flowers, spoon-feeding your garden with potassium supplements will keep your garden growing strong.


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Written by Harley Smith

Profile Picture of Harley Smith
Harley Smith is the director of research for NPK Industries. A veteran in the hydroponics industry, Harley has more than 18 years of consulting and educating experience. He is regarded as an expert on plant nutrition and organic bio-stimulants, performing research and new product development in the US and Europe.

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