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How To Identify, Treat, And Prevent Potassium (K) Deficiency Issues In Cannabis

Written by Sam North Mar 21 2023

One of the most difficult (and annoying) things that new growers have to come to terms with is the fact that nutrient deficiencies go hand in hand with growing cannabis crops – especially when you first start out. Deficiencies can quickly creep up out of seemingly nowhere, and identifying the root cause can be more than a little tricky.

Potassium (K) is one of the three main nutrients known as the macronutrients – Nitrogen (N), Phosphorus (P), and Potassium (K) – that all plants, including cannabis, need access to throughout the entire lifecycle. Potassium is responsible for promoting strong, deep root growth, and aids photosynthesis – all of which are essential to healthy growth, proper plant structure development, and flower maturation.

There is a very high chance that you will have to diagnose and treat a nutrient deficiency issue at some point during your first couple of crop runs, and potassium deficiency is one of the more frequently seen. Let’s take a deep dive into everything potassium related, and examine the causes of potassium deficiencies, how you can best treat the problem, and what you can do to prevent them from taking hold in the first place.

What Is Potassium, And Why Is It Important For Cannabis Crops?

Potassium is a mineral element that is naturally occurring in soil. The percentage of potassium found in a particular soil blend is dependent on the soil makeup, with clay-heavy soils containing the highest amounts – up to 4%. As one of the three main macronutrients in cannabis, it is essential for healthy growth and acts as an activator for several important enzymes that aid in the breakdown of carbohydrates, increasing metabolic activity and stimulating growth.

Potassium also helps plants to survive stress from environmental conditions. By strengthening the plant tissue and overall structure, potassium provides the plant with what it needs to create both sturdy and robust stems and branches. This extra strength and stability not only helps during times of extreme heat or cold but also increases the surface area for photosynthesis, giving cannabis plants access to more energy during the vegetative growth period. On top of this, potassium availability aids ganga plants by

  • Doubling down on photosynthesis, through the extra surface area for light absorption, and with the synthesis of carbohydrates.
  • Increasing the natural resistance to disease, infection, pests, and mold.
  • Improving the plant’s ability to uptake and effectively use water contained in the soil or substrate.
  • Massively boosting the size, potency, and density of the buds. 
  • Stimulating explosive early-stage growth.

The Causes of Cannabis Potassium Deficiency

This might sound a little backward, but the most common cause of potassium deficiencies in weed crops is actually down to the over-supply of fertilizers. One of the most common mistakes new growers make is thinking that the more fertilizer they add, the healthier the crop will be, which will result in bigger yields with higher potency.

The opposite is actually true – to a certain extent, anyway. When we, the cultivators, allow the levels of nutrients in the substrate to become overly saturated, the plant is unable to uptake what it needs – and if this continues over a long period of time, then deficiencies can occur. This is known as nutrient lockout and can affect not just potassium, but all of the nutrients that cannabis plants need to survive and thrive.

This is not to say that plants will do best when left totally unfertilized, not whatsoever. Think of it a little like the story of Goldilocks and the three bears – we have to make sure the nutrient levels are just right, not too high and not too low. If you are growing organically in a good quality soil blend this is less of a concern, as the microbes contained in the soil will regulate its own nutrient availability, however, for those growing in hydroponic systems it is essential to monitor and adjust the feedings on a regular basis, as the needs of the crop change as it develops.

In general, we tend to recommend plain water feeding for the first week or so of growth. Once the plant has entered full vegetative growth (once the plant has grown 2 nodes, or reached a height of around 12 cm) you can start feeding them with either organic or synthetic nutrients. For those growing with synthetic nutrients, start with a 1/4 dose of the bottle recommendation, and slowly bring it up to 1/2 dosage over the next few weeks.

Then there is the pH levels to think about. Again, this is less important for soil cultivators, but for anyone using hydroponic methods (that includes growing in coco-coir as your main medium), adjusting the pH levels of your water and nutrient solution is a must. This will ensure that the minerals are available for uptake, as certain elements like potassium become locked out when the solution is too acidic or alkaline. For hydroponic and soil-less crops, you want the pH of the water or nutrient solution to be between 5.5 to 6.5 (5.8 to 6.2 for the best results). Always test the pH last, after adding all the nutrients to the feed water.

What should you do if the pH is too high or low?

Just grab a bottle of either pH up or pH down from your local hydroponic supplier, or online. A few drops is all that’s needed to quickly bring even the largest nutrient solution reservoir into the correct pH range, and these products are totally non-harmful to cannabis crops.

Finally, plants can become potassium deficient due to a lack of root zone aeration, a problem that can occur with both soil and hydro grows, but it is much more common in soil based media. When the growth medium is overly compacted or fully saturated, it limits the oxygen flow to the root system which, in turn, prevents the plant from accessing the nutrients it needs. This can be avoided by amending your substrate with something like perlite or vermiculite (or even sand) to help improve drainage and aeration.

Signs And Symptoms Associated With Cannabis Potassium Deficiency

Thanks to the fact that potassium is a mobile nutrient, deficiency issues with this nutrient will show up in the oldest growth on the plant. It may present in a couple of different ways, but the most common is yellowing or browning between the veins (with the veins staying vibrant green), usually starting with only one or two of the larger, older fan leaves. If left untreated it will continue to spread until the entire leaf system is affected – and in extreme cases, it can even cause the yellowing of the buds themselves.

Potassium Deficiency

In some cases, the tips and edges of the leaves will turn brown and crunchy, which closely mimics the symptoms of light burn. This ‘burning’ will sometimes present as a slightly lighter browning, but yellowing is the most common sign. It can also present in a very similar way to nutrient burn, or overfeeding. The main difference between nutrient burn and a deficiency in potassium being that it is not only the tip that yellows and becomes crispy but the very edges of the leaves as well.

Another way it can present itself is unexpected and sudden stunted growth, as potassium helps to regulate the cell growth process. It can also show up as brown or rust-colored spots on the leaves, the leaves curling upwards and inwards on themselves, and as twisted leaf development.

How To Treat A Cannabis Potassium Deficiency, Or Prevent It Happening In The First Place

Once you have diagnosed the issue, it is time to roll up your sleeves and begin treating the deficiency. But as we just mentioned, identifying potassium deficiency can be a little difficult. So, to make things clear, first up we can eliminate any other potential causes.

Adjust The Ph Range Of The Root Zone

The first and most important thing to do is ensure that the root zone is in the optimal pH range. If this is off it can cause a number of problems and should be addressed before continuing with the treatment of the deficiency. To test the pH of the root zone, all you have to do is test the runoff water from the pots. As long as the water or nutrient solution and the runoff is in the correct range, then pH is probably not the issue. But, if the runoff is outside the optimal range, you will want to flush the plants.

Flushing is super simple – all you have to do is drench the roots (or medium) with water that has been pH regulated. You want to see plenty of runoff out of the bottom of the pots (25 to 50%) and then hit it again with the water after around 30 minutes.

Healthy weed plant roots

Once the flush has been completed, wait for a day or two for the soil or coco-coir to dry out, and then feed with an NPK-rich nutrient solution.

Check That The Lights Are At The Correct Height From The Canopy

Light burn can look a lot like potassium deficiency, so it’s important to make sure the lights are not too close. The actual distance that the lights should be hung is dependent on the type of light you are using. HID bulbs should be kept at a minimum of 35 cm (1 foot) above the plants, if not further. LED panels produce way less heat than HID options, but that doesn’t mean you can hang them super close to the plants. Always check the light distance recommendations that will be displayed on the packaging, and add an inch or two to be sure that the problem isn’t light burn.

Use Good Quality, Cannabis-Specific Nutrients

While buying cannabis-specific nutrients may seem like an unnecessary expense, the truth is that these nutrients have been formulated specifically to address cannabis needs – and they are designed to give your crop the best chance of producing huge yields of buds covered in the stickiest, dankest trichomes.

Once again, this is less important for organic soil crops, but if you are using a hydroponic or coco-based setup then it is definitely worth investing in some quality nutrients. We always recommend going with a two-part liquid nutrient regime, as these deliver a balanced and consistent supply of nutrients to the plants.

There is a veritable smorgasbord of cannabis-specific nutrient options available, but for the most part, we recommend sticking with the big names. Brands like Advanced Nutrients, CANNA, Botanicare, General Hydroponics, and CX Horticulture are all well-respected in the weed industry and have a great selection of both base nutrients and additives to choose from.

Cut Down On The Watering During The Vegetative Growth Stage

Over-watering is a common mistake for novice cultivators, but for the best possible results, it is advised to cut down the frequency of your watering. A wet-dry cycle (especially during the vegetative growth stage) can help promote a much larger root ball as the plants have to “stretch” out their roots in search of more water. This enlarged root ball promotes much healthier plants, and increased harvest sizes.

Using A PK Booster During The Flowering Growth Stage

Now then, we have gone over everything you need to know about treating and preventing a potassium deficiency. But you may have heard about the need for a PK booster during the flowering period. 

Is this necessary?

Probably.

As we have previously mentioned, adding an extra dose of potassium (in the form of a PK booster) during the flowering growth stage can be incredibly beneficial. It is important to note that you shouldn’t use this product during the vegetative stage as this will lead too nutrient toxicity (the opposite of a deficiency), but when implemented at the correct time a PK booster can work absolute wonders.

So, when should you use a PK booster?

The common consensus is that you should add a PK booster to your feeding schedule for around 7 to 10 days, 3 to 4 weeks before harvest. Most PK boosters are designed to work with all styles of cannabis cultivation, so it doesn’t matter whether you are growing in coco-coir, soil, or a hydro-system. Just mix them into your nutrient solution and apply as normal. In terms of how much to use, always refer to the manufacturer’s instructions for guidance on how to use their specific product.

Organic Potassium Sources That Can Be Incorporated Into Soil Or Coco-Coir Grows

While most of us do have access to high-grade, synthetic cannabis-specific nutrients and additives, there are many growers out there who like to keep things totally organic – fair enough!

There are a bunch of organic potassium options out there that can be used to boost the levels of this essential macro-nutrient in soil or coco-coir grows – with some of the more popular options include:

  • Compost – Most general, organic compost options will have a decent amount of potassium included, but if you are making your own compost (or compost tea), then try to include a bunch of banana peels or kelp meal to increase the levels of potassium.
  • Wood Ash – Also known as potash, the ash from any kind of wood provides a great source of potassium for organic gardens.
  • Seaweed Extract – Seaweed can be brewed into an extract and then added to the nutrient solution of your plants, providing them with an instant dose of potassium and other trace elements. 
  • Alfalfa Meal –  This is a great organic source of potassium and can be added directly to the soil or brewed into a compost tea.

How Dangerous Is Potassium Deficiency For Cannabis Crops?

If we were to rank the possible outcomes of nutrient deficiencies, then potassium would sit near the bottom of the pile. It’s rare that a plant will die from potassium deficiency-related issues, but that doesn’t mean that you shouldn’t always keep an eye out for potassium deficiency symptoms.

When left unchecked, a potassium deficiency can stunt the growth of your plants, reduce their flowering potential, and lead to an overall reduction in harvest size. And of course, if you don’t take action then your crop could eventually succumb to the deficiency.

So, to prevent any issues arising from a potassium deficiency, it is important to remain vigilant when it comes to nutrient levels, during both the vegetative growth stage and once the switch is made to flower production. Regularly check the condition of your plants, the pH and EC (or TDS) levels of your nutrient solution and plain water feeds, and if you do notice any symptoms beginning to form, then make sure that you take action immediately.

By following these simple steps, you should be able to fix any potassium deficiency issues quickly and efficiently.

Sam North

Sam North is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success. About this Author

Sam North

Sam North is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success. About this Author

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