Potassium And Cannabis Plants

Many nutrients are important for the healthy development of cannabis plants. In this post, we take a look at the functions of potassium. Read on to find out what role potassium plays in plant growth, and how you can spot, treat, and prevent deficiencies and toxic amounts of this essential nutrient.

What Is Potassium?

Potassium is a mineral element naturally found in soil. Clay soils are particularly rich in this mineral, and can contain up to 3% of it, but it is also found in other soils in lower amounts. Together with nitrogen (N) and phosphorus (P), potassium (K) is one of the three primary elements your cannabis plant needs in all growing stages to support vital biological processes. During flowering, cannabis has an extra high demand for potassium, where the element is important for the development of buds.

The Soil Potassium Cycle

Potassium self-regenerates in nature through what’s known as the potassium cycle. Plants take it up through their roots, and whatever remains in the land is washed out by rain and ultimately finds its way into the ocean. It then turns into water vapour, which condensates into rainfall, replenishing the soil. Another way the mineral returns to the land is through animal droppings. The cycle can then begin anew.

In regions with frequent rainfall, soil may not retain sufficient amounts of this mineral. Potassium in the form of sulphates or potassium chloride will then need to be added to make potassium available to plants.

Why Do Cannabis Plants Need Potassium?

As one of three primary macronutrients, potassium (indicated by the “K” in N-P-K, from Latin “kalium”) has many important functions in supporting the well-being of your cannabis:

• Strengthens plant tissue and provides sturdiness.

• Increases plant resistance to diseases and infections.

• Plays a part in photosynthesis, where it supports the synthesis of carbohydrates.

• Stimulates early growth.

• Increases resistance to frost. Potassium is used to “winterise” certain plants.

• Improves efficiency of water use.

• Increases bud size and density.

If potassium isn’t available in sufficient amounts (deficiency) or if there is an excess of it (toxicity), plants will get sick and yields will suffer. Potassium, like nitrogen and phosphorus, is a mobile nutrient. This means the plant can draw it from old growth and direct it to newer growth when the mineral is scarce. This is why signs of a potassium deficiency will often first show on older leaves at the bottom of the plant.

Potassium Needs During Early Stage

It is best not to feed seedlings and cuttings, as a lack of nutrients in the first 2–3 weeks of growth promotes the development of healthy roots. Wait until your plant has reached a height of 15cm before feeding. You can then start giving 25–50% of the recommended dose. Gradually increase to a full dose as your plant enters the vegetative phase.

Potassium Needs During Vegetative Stage

At about week 4, your cannabis plant has left the seedling stage and is now officially in the vegetative growth stage. At this point, your plant will require high levels of nitrogen along with other essential nutrients in appropriate amounts. When feeding commercial cannabis nutrients, stick to the recommended dose.

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Potassium Needs During Pre-Flowering Stage

Right before your cannabis enters the flowering stage, it will undergo a short pre-flowering period that will last for 1–2 weeks. The pre-flowering phase is like vegetative growth on steroids: Some cannabis plants will stretch considerably, sometimes two or three times their current height. To support the plant during this time of vigorous growth, you'll want to provide extra levels of nitrogen, potassium, and calcium.

Potassium Needs During Flowering Stage

During flowering, your cannabis plant will require extra potassium. You will normally increase potassium levels at the same time, or right after, increasing phosphorus and decreasing nitrogen at the end of pre-flowering when the plant has stopped growing. From there on out, the plant will be entirely focused on bud production. Extra phosphorus and potassium will now be important as these minerals play an important role in bud size, flavour, and overall development.

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TIP: Commercial cannabis nutrients are often available in two variants: one bottle for vegetative growth and another for flowering. Each typically contains the proper nutrient ratios for each phase—i.e. the flowering formula will contain less nitrogen (N) and more phosphorus (P) and potassium (K). The cannabis industry standard for N-P-K during vegetative growth is 3-1-2. For flowering, it’s 1-4-5.

Potassium in Chemical vs Organic Fertiliser

Animal manures and plant residues such as hay and straw are good potassium fertilisers. But there are other natural sources of this mineral. In fact, when using these natural materials, they will supply enough potassium to negate the need for an inorganic potassium fertiliser. To avoid potassium toxicity, proceed with caution when adding sources of potassium to your soil.

Good sources of potassium include:

• Compost: Compost, especially when amended with banana peels and other fruit and vegetable waste, is an excellent source of potassium and other nutrients. The potassium here is water-soluble and immediately available.

• Kelp meal: Kelp and seaweed, which are available as liquids or in dry form, are rich in readily available potassium.

• Wood ash: Wood ash, aka potash, is the classic potassium fertiliser. Hardwood ash can be directly added to soil or it can be added to a compost pile to increase potassium content.

• Cocoa shells: Available for use as mulch, cocoa shells are naturally rich in potassium.

• Muriate of potash (potassium chloride): This is widely available and makes a good natural source of potassium. However, the chlorine in it harms soil microorganisms, which makes it less suited for organic growing.

• Sulphate of potash (potassium sulphate): More suitable for organic growing than muriate of potash as it doesn’t contain chlorine.

• Granite dust: Less suited as a quick fix as it releases potassium very slowly. But it makes a good and inexpensive soil amendment that also provides other minerals.

• Greensand: This is mined from ancient former sea beds and contains potassium and other minerals. Flexible in how it can be used, greensand can be mixed with compost or added directly to soil.

Potassium Deficiency

Potassium deficiencies are relatively common. If poor soil quality can be excluded (such as when using commercial soil made for growing cannabis), the problem is almost always nutrient or pH related.

What Causes Potassium Deficiency?

• pH level out of range: Potassium has an optimal pH range between 6.1 and 7.1. If the pH of the soil or water/nutrient solution is above or below that, potassium won’t be available to the plant.

• Poor soil: Soil that isn’t suitable for cannabis, or is low-quality, will likely be deficient in potassium.

• Giving too much calcium: Too much calcium can lock out potassium and magnesium.

• Not providing adequate amounts of nutrients: Not all cannabis strains have the same nutrient requirements. A feeding regimen that works for one cannabis variety may not provide enough nutrients for another.

• Salt/mineral accumulation in pots: Over time, mineral salts from feeding will accumulate in growing containers and can throw off the pH level at the root zone. When this happens, potassium lockout will occur. A flush will be in order to restore pH to a healthy level.

How to Recognise Potassium Deficiency

Because potassium is a mobile nutrient, symptoms of a deficiency will usually first show on older foliage, but not always. Symptoms include:

• Leaves turn yellow from the tips towards the centre.

• Leaf edges turn brown and crumble.

• Older leaves turn yellow.

• Plants appear weak with stems that bend easily.

• Significantly decreased yields.

TIP: Potassium deficiency may appear like other problems, such as nutrient burn or light burn. However, the discoloured tips from potassium deficiency go further than from nutrient burn, plus there is yellowing between the leaf margins. Make sure to also exclude light burn, which would only show on the leaves closest to your lights.

How to Treat Potassium Deficiency

Whatever the reason for your potassium deficiency, once diagnosed, it’s time to get to work getting potassium levels back on track. Here’s how to do it according to the substrate you’re growing in.

• In Soil

  If using mineral nutrients, increase EC for a couple of feeds to fix. When growing organically, add soil amendments (wood ash, sulphate, cocoa shells, etc.) to increase potassium levels. Ensure water/nutrient pH is between 6.0–7.0 pH.

• In Hydro/Coco

  Increase your EC level, but do so gradually. It’s best to increase the EC of your feed by 0.2 points every few days. Carefully observe your plants and stop increasing EC when they start looking healthy again. Also, make sure you use bloom nutrients as these have higher potassium levels. Check your pH levels. For hydro and soilless operations, you want a pH of 5.8–6.2.

Potassium Toxicity

Potassium toxicity (excess) is more likely to happen than some other nutrient excesses. Causes include overfeeding, poor soil, or using soil amendments that contain too much potassium.

How to Recognise Potassium Toxicity

Potassium excess is not always easy to diagnose as it can resemble other nutrient problems. Symptoms include:

• Lower leaves spotting and curling.

• Leaf tips and margins develop yellow/brown “burn” and dry out.

• Top leaves grow thin blades.

• May resemble other deficiencies (calcium, magnesium, etc.)

How to Treat Potassium Toxicity

Treating potassium toxicity is largely a matter of getting pH and nutrient levels back on track.

• In Soil

  Flush medium with pH-balanced water. Stop feeding for one week and allow the plant to use up the remaining potassium.

• In Hydro & Coco

  In hydro, flush and reduce EC in tank; change water in tank. In coco, flush the substrate and reduce EC level.

How to Prevent Potassium Issues

• Use good soils from reputable brands as these are tested for their mineral levels.

• If using natural soils and/or experimenting with soil amendments, test your soil frequently. If this is not an option, keep a close eye out for signs of deficiency/toxicity.

• Stick to recommended feeding doses and schedules.

• Know the nutrient requirements for your particular cannabis strain. Some strains are light/heavy feeders.

• If problems with potassium persist, consider using hydroponics as opposed to soil. Potassium deficiency is less likely to occur as the plants can more easily uptake available nutrients.

Luke Sholl

Luke Sholl has been writing about cannabis, the wellness potential of cannabinoids, and the positive influence of nature for over a decade. Working with several cannabinoid-centric publications, he publishes a variety of digital content, supported by strong technical knowledge and thorough research.

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