Causes of Potassium Deficiency in Plants, and How to Fix It


The other day, I was researching various nutrient deficiencies in plants (my favorite hobby, obviously).  I began to wonder how potassium deficiency can occur in plants, especially when so many fertilizers contain plenty of potassium.

So, what causes potassium deficiency in plants?  Either there is not enough potassium for the plant, or something prevents the plant’s uptake of potassium.  Incorrect soil pH, dry soil, and extreme temperatures can all prevent a plant’s uptake of potassium.

First, let’s talk about why potassium is important for your plants.  Then, we’ll look at the symptoms of potassium deficiency and how to recognize them.  That way, you can correctly identify the condition before you try to solve a problem you might not even have!  Finally, we’ll take a look at each of the causes of potassium deficiency, along with ways to treat each of them.

Why is Potassium Important For Your Plants?

Potassium serves several important functions in a plant.  First of all, potassium is involved in photosynthesis, which produces energy and chlorophyll for the plant.  Potassium also regulates the growth rate of the plant.

In addition, potassium helps to transport water and nutrients throughout the plant.  Finally, when a plant has enough potassium, this contributes to strong roots and stems.

These sound like amazing benefits for a plant, but what happens when a plant doesn’t get enough potassium?  It isn’t pretty – let’s check it out.

Symptoms of Potassium Deficiency

The symptoms of potassium deficiency are reduced growth, yellowing of old leaves, weak roots, small stems, defoliation, and low resistance to extreme temperatures and drought.

In addition to looking out for the symptoms below, you can test your soil to know for sure if you have a potassium deficiency. For more information, check out my article on how to do a soil test.

Reduced Growth

If you see one plant or group of plants in your garden that are smaller than the others, then a potassium deficiency may be to blame.  Remember that sufficient potassium helps to regulate a plant’s growth rate.

A lack of potassium, on the other hand, will slow down the growth rate, since the plant is unable to effectively transport nutrients and water throughout its tissues.

Insufficient potassium can also decrease the effectiveness of fertilizers containing nitrogen, according to the potash development association website.

Yellowing of Old Leaves

Chlorosis, or yellowing of leaves in a plant, is caused by a lack of chlorophyll, the compound that makes plants green.  As mentioned earlier, potassium is involved in plant photosynthesis.

chlorosis
These leaves are showing interveinal chlorosis – note how the veins are still green.

When a plant suffers from a potassium shortage, chlorophyll production will slow down, causing leaves to become yellow-green and eventually yellow.

When a potassium deficiency occurs, older leaves (those that are lower on the plant) will turn yellow first.  This is because potassium is a mobile nutrient, meaning that the plant can easily transport it throughout its tissues.

Since the plant can transport potassium, it will prioritize newer leaves, which are higher on the plant and likely to collect more sunlight for photosynthesis.

Weak Roots

Without enough potassium, the root system of a plant will be much weaker.  This decreases the plant’s ability to take in both nutrients and moisture from the soil.  So, a lack of potassium can indirectly lead to a shortage of other nutrients, or even water.

Small Stems

Sufficient potassium helps to ensure that the stems of a plant are strong.  Without enough potassium, the stems become weak and spindly. 

Defoliation

Remember that in the flowering stage, your plants will need more potassium than in the vegetative stage.  If there is not enough potassium, the flowers may fall off of your plants.

If the deficiency is severe enough, the plant may not develop flowers at all.  However, you will probably be aware of the deficiency before this point, due to the other symptoms mentioned earlier.

If you are lucky enough to get flowers at all, you may not get any fruit from the plant.  If you do, the production will be reduced, and the fruits may be small.

Defoliation means that the plant has fewer leaves to help absorb sunlight for photosynthesis.  This leads to reduced energy production and storage.  Defoliation also means less shade for the plant, which increases the risk of burned leaves or overheating on a sunny day.

Low Resistance to Temperature and Drought

All of the symptoms mentioned previously contribute to a plant’s decreased resistance to temperature and drought.

With weak roots, the plant is unable to go deeper into the soil to find water when there is no rain.  Stunted growth means that the plant has smaller reserves of nutrients, water, and energy to see it through a crisis.

drought
This looks like a pretty serious drought.


Causes of Potassium Deficiency in Plants

These symptoms of potassium deficiency sound pretty bad, and that’s because they are.  So, what are the causes of these symptoms?  Let’s get right into that.

Lack of Potassium in Soil

This is perhaps the simplest explanation of potassium deficiency in plants, but it is not always the correct one.  You may have a potassium deficiency if you neglect to use crop rotation in your garden.  Planting the same crop in the same place, year after year, will deplete the nutrients over time – including potassium.

If you are seeing symptoms of potassium deficiency for the first time in your garden, then supplementing with a fertilizer high in potassium will help to improve the soil quality and the health of your plants.

Fertilizers are labelled with three numbers, denoting “NPK”, or nitrogen, phosphorus, and potassium.  For example, a 10-10-10 fertilizer contains 10% nitrogen, 10% phosphorus, and 10% potassium by weight.

It is possible to find fertilizers with a content skewed towards higher potassium content. For more information, check out my article on low-nitrogen fertilizers.

potash
Potash, with a penny for scale. Potash is used in many commercial fertilizers.

Some common potassium supplements without nitrogen or phosphorus are potassium chloride, potassium sulfate, potassium-magnesium sulfate, potassium thiosulfate.  You can use the sulfates if your plant also needs a sulfur boost.  You can use potassium nitrate to provide potassium and nitrogen together.

You can also create your own potassium-rich compost by putting banana peels, coffee grounds, and ashes from a wood-burning stove into a compost mixture.  Then, you can use this to supplement potassium for any part of your garden that is lacking.

One more important note: it is possible to have too much potassium in your soil as well.  If you have been adding fertilizer with potassium to your soil, then you should seriously question whether you have a potassium deficiency.

As mentioned above, potassium deficiency will affect old leaves first, but excessive potassium will affect new leaves first.

It is entirely possible that your plants will still exhibit signs of potassium deficiency, even if there is plenty of potassium in the soil.  Let’s look at how that can happen, and how you can prevent it.

Incorrect Soil pH

One potential cause of a potassium deficiency in your plants is incorrect soil pH.  A good general pH range is 5.5 to 6.5 for most plants.

If the pH is much higher or lower than this, you will start to have problems.  The reason is that the availability of each nutrient depends on the soil pH, as you can see in this pH diagram from the Research Gate website.

As you can see, potassium is highly available at a soil pH all the way up to 10.0 (highly alkaline soil).  However, when soil pH drops below 5.0, the availability of potassium in the soil drops off significantly.  If the soil pH goes as low as 4.0, potassium will have low availability.

To find out your soil pH, you will want to pick up a soil pH meter, available at Home Depot, Lowe’s, Amazon, or your local garden store.  If your soil pH is too low, it may be preventing your plant from absorbing potassium from the soil. For more information, check out my article on how to test your soil pH.

One way to raise the pH of your soil is to add lime, which contains calcium, to your soil.  For more information, check out my article on how to raise your soil pH. Another method is to mix wood ash into your soil.

Coincidentally, wood ash will also add potassium to your soil, as mentioned earlier.  So, if you’re not sure whether the problem is lack of potassium or low pH (acidity) in the soil, then wood ash is a good all-purpose fix to try.

Dry Soil

Dry soil that drains easily can also decrease the availability of potassium for plants, even if you add fertilizer to supplement potassium. For more information, see my article on how to treat dry soil.

Remember that potassium is highly soluble in water, and so it is easily dissolved and washed away, especially during rainy seasons or heavy watering.

dry soil
This soil is dry and cracked due to lack of water. This can cause potassium deficiency in plants.

To make potassium more available to your plants, keep the soil moist without flooding it.  If you see water carrying away some of the soil around your plants, then you may be overdoing it.

Try pouring a bit of water on each plant at first, making your way down the row.  Then walk back to the beginning of the row, and repeat the process.

Also, make sure to do your watering in the morning, when temperatures are cool and the sun is low.  This will prevent the water from evaporating quickly in the sun and heat.

Extreme Temperatures

The ideal temperature range for uptake of potassium is 60 to 80 degrees Fahrenheit.  For more information, check out this article by the University of Minnesota Extension.

A Few More Words About Excessive Potassium

One final thing to remember here: gardening is all about balance.  You need to have enough of each nutrient in your soil, but you also need to have the right balance of nutrients.  An excessive amount of one nutrient can prevent a plant from taking up another nutrient.

For example, too much potassium can cause a deficiency of other nutrients in your plant.  Increased potassium can decrease available calcium and magnesium.  Likewise, increased calcium can also decrease available magnesium.

As you can see, the interplay between nutrients is complex – and this is only three nutrients out of over a dozen needed for plant growth!

Conclusion

By now, you have a better idea of what signs to look for to identify potassium deficiency in plants, along with some of the potential causes.

I hope this article was helpful.  If you have any questions, please leave them in the comments below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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