September 24, 2022

Cannabis magnesium deficiency: how to identify and treat a cal mag deficiency

Cal mag (calcium magnesium) deficiencies are a common issue for sick cannabis plants. These deficiencies can stunt plant growth and turn leaves brown and yellow, gradually killing off your entire plant. Cal mag supplements are a popular treatment for this problem.

However, it’s also important to treat the root cause for any deficiency. Mistreatment of a cal mag deficiency will fail to solve the problem, and can actually make matters worse. In this article we will discuss how to identify, treat, and prevent cal mag deficiencies.

How to tell if you have a cal mag deficiency

Although a cal mag supplement can help treat both deficiencies, calcium and magnesium deficiencies are two separate issues. Both of these deficiencies have their own set of symptoms.

This is due to nutrient mobility in the plant. Cannabis plants treat and process these two nutrients very differently when they absorb them.

Magnesium deficiency symptoms

Since magnesium is vital for chlorophyll production (what turns your plant green), a magnesium deficiency will cause leaves to begin turning yellow.

During a magnesium deficiency you may see the following symptoms:

  • Leaves start to yellow on old growth first. Especially for leaves and growth at the bottom of the plant
  • Leaves first become light green (with brown spots) as chlorophyll production begins to halt
  • Leaf veins will often remain green
  • Color change starts at the edges of the leaves, which can also become rough to the touch
  • Leaves eventually turn fully yellow and begin to fold in on themselves.
  • Leaves that dies don’t fall off on their own
  • As the magnesium deficiency continues, yellow leaves will spread up the cannabis plant

Magnesium is an mobile nutrient. This means that plants can easily transport it to different parts. Cannabis plants will always prioritize new growth during a magnesium deficiency. It will move magnesium from old growth to support the new. It will deprioritize old growth, allowing it to die.

Calcium deficiency symptoms

Calcium deficiencies usually result in the following symptoms:

  • New growth, particularly at the top of the cannabis plant, will turn spotted and brown
  • Cannabis plants with a calcium deficiency will have their new growth completely stunted
  • New buds will be small and stop growing
  • Cannabis plant roots will also have slow growth, limiting the size of the plant

Calcium is a immobile nutrient. This means that once a plant absorbs and uses it, that nutrient will stay in the same part of the plant. Plants require calcium for new growth. Since it is an immobile nutrient, the plant cannot move existing calcium to support the new growth. New growth always requires the plant to absorb more calcium.

Why cal mag supplements are a popular treatment

Cal mag supplements are popular because calcium and magnesium (along with iron) can be easily combined into one supplement. This makes the supplement a popular catchall for treating several deficiencies at once, especially if the grower is not certain which deficiency their cannabis plant has.

Magnesium and calcium deficiencies will commonly show up together in plants. This is because cannabis plants that have trouble absorbing one mineral may have trouble absorbing other minerals as well.

It’s important to identify any root cause before you treating your plant with a cal mag supplement, because other issues commonly cause these deficiencies.

What causes cal mag deficiencies

There are several potential causes of a cal mag deficiency. Calcium and magnesium are usually readily available in the water supply, so cal mag deficiencies are usually caused by outside factors.

Common issues include issues with root absorption problems, and also overtreatment of the water used on the plants.

Low root Ph

This is one of most common causes of cal mag deficiencies. The roots of cannabis plants require a soil ph of 6.0-7.0 (5.5-6.5 for hydroponic grows) to properly absorb nutrients.

If the root ph begins to drop, the root system will become worse and worse at absorbing nutrients from the ground. Below a ph of 5.0, the plant will be unable to absorb any nutrients from soil.

It’s important to properly diagnose your soil ph, because even if nutrients are present, your plant will not absorb them if the soil has an improper ph level.

Too much moisture

Another root system issue for cannabis plants is that they can have too much moisture in the soil. This can adversely affect the roots, making it difficult for them to absorb nutrients.

Root systems require periods of dryness to properly absorb oxygen from the soil. If they fail to absorb enough oxygen, the root system can begin to fail.

Finally, overwatering a plant can lead to root rot. Root rot is a serious issue that is difficult to treat and can kill your entire plant.

To keep moisture and proper levels, only water your plant when the top inch or so of soil has dried out. Only water your plant until you see a small amount of water drain out the bottom of the pot. Also, make sure that your growing pot and soil allow for proper drainage.

Cal mag deficiency using coco coir

Coco coir is a popular growing medium for hydroponic systems. This growing medium is inert, meaning that it contains no nutrients itself. Instead, minerals are provided via a nutrient solution.

Coco coir has a tendency to lock out calcium from plant absorption. So any calcium in your plant food may not be present at high enough levels to be absorbed. In this case, it may be necessary to provide your plant with extra calcium. Cal mag supplements work great for this issue.

Using reverse osmosis, heavily purified water, or soft water

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Some growers believe it is better to water their plants with heavily purified water. The problem with this approach is that it may remove vital nutrients and minerals from the water supply.

If this is causing your cal mag deficiency, consider using water that has not been purified. Otherwise, it may be necessary to provide additional nutrients through a cal mag supplement.

If your area has soft water, then the water will naturally have less minerals in it. If this is the case, it may be necessary to supplement minerals to make up for the deficiency.

Overload of other nutrients

It’s possible to feed your plant too much fertilizer. In addition to causing issues such as nutrient burn, it these excess minerals can also compete for absorption with magnesium. In such a case, the other nutrients may “lock out” magnesium from being absorbed by the roots.

If you suspect this may be the case, try flushing out your soil with proper ph water. Then, hold off on supplying more fertilizer until the problem resolves. A cal mag supplement can also be used to quickly recover the plant.

High humidity levels

Humidity levels are a less common cause of a cal mag deficiency, but can still contribute to the issue. High humidity can affect cannabis plants, making it harder for them to shuttle nutrients to needed areas.

It could potentially be the case that the plant roots are absorbing the nutrients, but the plant cannot move them to the correct area.

If you suspect the may be the issue, try to lower your humidity levels in your grow room, either through improved ventilation or by using a dehumidifier.

When to use a cal-mag supplement (calcium magnesium supplement)

Cal mag supplements can treat a cal mag deficiency, but only if any other outside problems have been corrected. For example, we previously discussed how issues such as soil ph and overwatering can prevent roots from properly absorbing nutrients. If these issues are present, no amount of of cal mag supplements can recover your plants.

However, if any underlying issue have been fixed, cal mag supplements are a great way to recover your plant. While it’s possible that the plant will absorb these nutrients through the water supply, a cal mag supplement or nutrient solution can give your plant the boost it needs to begin recovering.

Cal mag supplements are also good to use in situations when calcium or magnesium are not readily available. This includes for growers using a coco coir growing medium, or for growers in areas with soft water or who insist on heavily purifying their water.

Please remember that a cal mag supplement can only remedy existing deficiencies, and only if all related problems have been solved. It cannot make your plant grow faster, bigger, or stronger.

Cal-mag supplement alternatives

While cal mag supplements are a popular solution for treating these deficiencies, other options exist as well. The most popular of these are epsom salts and dolomite lime.

Epsom salts

These salts are very popular with gardeners of all kinds. Epsom salt contains both sulphur and magnesium. Since it’s a salt, it can easily be absorbed in water and then fed to the plant through the soil or through foliar feeding.

Foliar feeding is the process of spraying the nutrient solution directly onto the leaves. Since a magnesium deficiency usually shows up in the leaves first, foliar spray is a popular method to help plants quickly absorb magnesium.

Epsom salt doesn’t contain calcium, so if you are concerned about a calcium deficiency as well, try mixing some calcium nitrate fertilizer into the soil.

Using foliar spray

  1. Heat the water to improve absorption
  2. Mix in a teaspoon of epsom salt for every liter of water
  3. Wait for the solution to cool back down
  4. Put the solution in a sprayer and spray it on the leaves

Dolomite lime

Dolomite lime is a mixture of calcium and magnesium, which works well for treating cal mag deficiencies. It is applied to plants by mixing it into the soil, and has a ph of 7 which can help raise soil ph if it is too low.

Dolomite lime works well over time, because because a plant can absorb it gradually. While it is a good long term solution, growers who want immediate results may decide to use epsom salts or a cal mag supplement.

How much cal mag supplement should you feed to your plants?

Most cal mag supplements have the recommended feeding levels listed on the supplement itself. It may be good to use levels slightly under the recommended feeding amounts, as that is usually the maximum you want to use. Excess minerals don’t benefit the plant and can disrupt soil ph or lockout other nutrients.

If making your own solution, aim for a concentration of around 380ppm, with 260 ppm calcium and 120 ppm magnesium. This should be a reasonable amount to feed your plant.

How to tell if your plant has recovered

A cal mag deficiency takes at least a couple of days to start resolving. The most common sign that a magnesium deficiency has resolved it that the leaf yellowing stops increasing. While the yellowing will stop increasing, these leaves will never revert back to green.

Be sure that the magnesium deficiency has resolved before removing any of these yellow leaves. If the magnesium deficiency has not resolved, the plant will begin to rob magnesium from other older growth to support the new growth again, and new yellow leaves will appear on the remaining older growth.

When a calcium issue resolves, the plant will begin growing again at a speedy rate. Brown growth due to a lack of calcium is not fixable, and a grower should prune or remove any bad growth from the plant.

Why calcium and magnesium deficiencies are harmful to plant growth

Cannabis plants require calcium and magnesium in trace amounts for healthy growth. Deficiencies in either of these minerals will bring your plant’s growth to a complete halt.

What calcium and magnesium do for your plant

Cannabis plants use these minerals, along with iron, in many different processes vital to plant health. These include:

  • creating chlorophyll for photosynthesis
  • metabolizing potassium and phosphorus for bud production
  • maintaining the structure of plant cells

Plants with a cal mag deficiency will fail to produce the energy they need through photosynthesis. This will stop them from growing bigger or even maintaining their current health.


Calcium and magnesium are vital to your plant’s growth and overall health. A deficiency in either mineral should be fixed quickly, it can yellow leaves and stunt new growth. Calcium and magnesium occur naturally in water, so a deficiency is usually caused by outside factors. It’s important to diagnose issues such as a ph imbalance or overwatering before trying to use a cal mag supplement. These supplements, however, are good for helping to recover cannabis plants.

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