Magnesium And Cannabis Plants

Magnesium is a vital nutrient that cannabis needs for healthy growth and large yields. Find out the functions of magnesium in cannabis, and learn how to spot and prevent a deficiency.

Magnesium And Cannabis Plants

Magnesium is an essential nutrient that your cannabis requires in relatively large amounts. If there is a lack of it, or, less commonly, too much of it (magnesium toxicity), your plant will get sick, and may even die. We’ll take a look at magnesium and its role in the growth of your cannabis. We’ll also show you how to spot magnesium deficiency and toxicity, and how you can treat and prevent either occurrence.

What Is Magnesium

What Is Magnesium

Magnesium is among the important secondary nutrients that cannabis plants need for healthy growth. Like nitrogen, phosphorus, potassium, calcium, and sulphur, it belongs to the group of macronutrients, which are the nutrients your cannabis needs in the largest amounts.

Magnesium is classed a “secondary” macronutrient because, although needed in relative abundance, less is required for your plants to grow compared to the three primary macronutrients: nitrogen (N), phosphorus (P), and potassium (K). But, just like the primary nutrients, secondary nutrients are essential to the growth of your cannabis.

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Magnesium is also a so-called mobile nutrient. This means the cannabis plant can relocate it from older tissue to newer tissue. This is why signs of a magnesium deficiency usually show up on the older, lower leaves first, and not on new growth.

Why Do Cannabis Plants Need Magnesium?

Why Do Cannabis Plants Need Magnesium?

Magnesium is an element that your cannabis plant needs in relatively large quantities in all stages of its life. Within the plant, magnesium is mostly present in the leaves, as it is a vital part of the chlorophyll molecule. Its presence or absence, therefore, has a direct impact on the plant’s ability to absorb light and subsequently create sugars and carbohydrates. If magnesium levels are low, your plant will struggle to grow. In addition, magnesium also helps with enzyme production.

In summary, magnesium:

  • Plays an important role in converting light into energy.
  • Regulates enzyme production.

In some circumstances, cannabis may require additional levels of magnesium: for instance after supplementing with something that contains calcium but not magnesium, such as eggshells or agricultural (non-dolomite) lime. Overfeeding with cannabis fertilisers can at times also result in a deficiency due to an overly acidic pH in the root zone. In this event, simply giving magnesium won’t work, as the reason for the deficiency is an improper pH, rather than a lack of magnesium itself.

Magnesium Needs During Early And Vegetative Stage

Seedlings and cuttings do not require nutrients. In fact, not providing nutrients in the first few weeks of growth will promote healthy root development. Only start feeding when your plants have reached a height of 15cm, and then go about it gently and start giving only 25–50% of the recommended dose.

When your plants have entered the vegetative stage at about week 4–8, you can start giving nutrients at the recommended doses. At this stage, where your plants grow vigorously and expand their root system, they will need to be fed nutrients rich in nitrogen, along with the proper amounts of magnesium, calcium, and other macro- and micronutrients. If you follow a feeding schedule and stick to the recommended doses, your plants should get all the required nutrients in the appropriate concentrations.

Magnesium Needs During Pre-Flowering And Flowering Stage

The time right before cannabis enters the flowering phase is known as pre-flowering. This is where the plant grows most vigorously, with some strains stretching twice or even three times their current height within just a week. To accommodate this vigorous growth, plants will require nitrogen and extra levels of magnesium and calcium.

Because magnesium is essential for cannabis to turn light into energy, not getting enough will also negatively affect bud growth and yield. It is therefore best to give flowering nutrients in the recommended doses. This will ensure your plant receives the right amount of magnesium and other essential nutrients to develop an abundance of buds.

If you have amended your soil with eggshells or agricultural lime (both of which provide calcium but no magnesium), or your tap water has a high amount of lime, you may need to give extra magnesium to avoid deficiencies.

Cannabis, Magnesium, And Ph Level

Cannabis, Magnesium, and pH Level

As with most other nutrients, cannabis plants can only absorb magnesium within a specific pH range. When growing in soil, magnesium is available from 6.5 to 8.0 pH. With hydroponics, the suitable pH for magnesium is 5.8 to 8.0 pH. A common reason for magnesium deficiency is an overly acidic pH. This will lead to magnesium lockout, meaning the plant is unable to absorb magnesium even if it is present. When this happens, the grower needs to restore pH to an appropriate level. This is done by flushing out the root zone with plain pH-balanced water.

Magnesium In Chemical Vs Organic Fertiliser

Magnesium in Chemical vs Organic Fertiliser

As it relates to cannabis plants, there are two types of magnesium. One is the mineral in ion form, such as magnesium oxide, which is found in cannabis nutrients and sometimes in soil. There is also unavailable magnesium that doesn’t get processed by microbes, making it so plants can’t absorb it. Soil for growing cannabis will typically contain both readily available magnesium and magnesium that needs to be first metabolised by microbes.

If one is growing organically and doesn’t want to use chemical nutrients, Epsom salt is a great natural alternative that usually isn’t chemically derived. As a soil amendment that resembles coarse kitchen salt, it gives your cannabis an extra boost of both magnesium and sulphur. It’s also almost impossible to oversaturate the soil when using it, so you won’t have to worry there. Epsom salts work in hydroponic systems as well. Lastly, dolomitic lime is another natural source of both magnesium and calcium, and it makes a great soil amendment.

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Magnesium Deficiency

Magnesium deficiency is a relatively common problem when growing cannabis. Fortunately, it is relatively easy to recognise and treat. Signs and symptoms of a deficiency will usually show on older and “middle-aged” leaves.

What Causes Magnesium Deficiency?

There are various causes behind a magnesium deficiency:

  • High levels of calcium, potassium, or ammonia—often from too much lime in tap water, or from using clay soils rich in chalk.

  • Excess moisture, cold temperatures, or acidic pH at the root zone.

  • Plant stress (excessive temperatures, intense grow lights, etc.).

  • Too high an EC in the growing medium (e.g. from overfeeding or incorrect nutrient dose).

  • Insufficient moisture evaporation or poor ventilation in the grow room.


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How To Recognise A Magnesium Deficiency

If your cannabis plant is lacking magnesium in its substrate, it will draw magnesium stored in its leaves and “send it” to newer growth. This is why the first signs of a deficiency will usually show on the lower or middle fan leaves. Only at the later stages of magnesium deficiency will there be symptoms on new growth.

Early stage:

  • Edges of older leaves turn yellow and crispy.
  • Margins inside the leaves lighten and turn yellow, but interveinal space will stay dark.
  • Can manifest as reddish or purple stems, although this sign alone isn’t necessarily indicative of an mg deficiency.

Late stage:

  • As magnesium deficiency progresses, brown spotting within the leaf margins or along the edges can occur.
  • Without treatment in the later stages, symptoms such as necrotic spots can also show on new growth.

How To Treat Magnesium Deficiency

How to Treat Magnesium Deficiency

Here is how to treat a magnesium deficiency depending on your growing method (soil, coco, hydro).

  • In Soil

    Make sure soil/water pH is at least 6.0, although the optimal level for magnesium is above 6.5. If soil is too acidic (below 6.0), flush (wash out) soil with plain water at the correct pH. Afterwards, give nutrients in the proper dose at the optimal (6.5–7.0) pH. Use a foliar spray with calcium and magnesium as a quick first-aid to directly supply magnesium to your plant.

  • In Hydro/Coco

    If growing in hydro, check ppm levels in your tank. If below 150, add magnesium and calcium (CalMag). If water pH is below 5.8, flush your system with pH-balanced water and provide nutrients at the correct levels. As an added emergency measure, use a CalMag foliar spray to directly provide magnesium to your plant. If growing in coco, make sure pH is in range. If pH has dropped too far, flush the substrate and then supply nutrients in the recommended dose. Foliar feed CalMag as a first aid to help your plant recover.

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Magnesium Toxicity

Magnesium toxicity (excess magnesium) is very rare. But should it happen, it will severely affect the uptake of nitrogen, which is crucial for your plant to grow strong and healthy.

How To Recognise Magnesium Toxicity

Magnesium toxicity isn’t easy to diagnose as it can mimic other plant problems. One of the symptoms, however, is leaves turning a dark brown-black colour. Magnesium toxicity also affects the absorption of calcium in the plant, leading to calcium lockout. As a result, magnesium toxicity will also show some or all of the symptoms of calcium deficiency.

How To Treat Magnesium Toxicity

How to Treat Magnesium Toxicity

How to best treat a magnesium toxicity depends on if you’re growing in soil, hydro, or coco.

  • In Soil

    If you suspect excess magnesium in your soil, flush plants with three times the container capacity of 6.5–7.0 pH water. This will get rid of excess minerals. Following that, do not feed for one week and give only pH-balanced water. Afterwards, give nutrients in the recommended amount.

  • In Hydro & Coco

    Flush medium with pH-balanced water. If growing hydroponically, change water in your tank. Start feeding at half the normal EC.

How To Prevent Magnesium Toxicity

How to Prevent Magnesium Toxicity

While magnesium toxicity rarely occurs, the key to preventing it is to keep an eye on nutrient levels. This doesn't just mean the nutrients you're feeding to your plants at a given time, but also the minerals that may already be present in your soil.

  • In Soil

    Give nutrients in the recommended doses and don't overfeed. Also, use ready-made soils for growing cannabis. Avoid growing in soils from unknown sources where nutrient levels have not been analysed.

  • In Hydro & Coco

    Administer recommended level of nutrients. If growing in hydro, monitor EC/ppm levels in your tank frequently. Reduce feed if ppm is too high.