How to Fix Magnesium Deficiency in Cannabis Plants?

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Added 21 November 2022

Growing a cannabis plant is like juggling various balls while new balls are being thrown at you — it can get a little overwhelming for many new growers. In this struggle of juggling various elements of plant growth, we often overlook the minor stuff, i.e., micronutrients.

However, one of the micronutrients that is essential for your plant’s growth more than others is magnesium. Without it, your plant will essentially starve and eventually die off. But how do you identify magnesium deficiency in cannabis plants and fix it before it claims your plant? Follow this guide to know all about magnesium deficiency in cannabis. 

Why is Magnesium Essential for Cannabis?

Why is Magnesium Essential for Cannabis?

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While growing, cannabis needs a variety of nutrients, including the three crucial macronutrients, nitrogen, phosphorus, and potassium, along with various other micronutrients. Out of the micronutrients, magnesium is perhaps one of the most important for healthy plant growth.

And while magnesium is a micronutrient, cannabis still requires it in relatively large quantities throughout its growth. 

The primary duty of magnesium in cannabis plants is in the leaves — it is an essential building block of chlorophyll, which is responsible for photosynthesis. Magnesium’s presence in the leaves is directly proportional to its photosynthetic abilities, without which, the plant would fail to produce carbohydrates and sugars.

So, if your plant does not get enough magnesium, it will eventually starve and die as it cannot absorb light and turn it into energy efficiently. 

Magnesium is also crucial for the roots — it helps the roots absorb other nutrients more efficiently, especially nitrogen and phosphorus. Without adequate magnesium, cannabis can suffer other forms of nutrient deficiencies. 

And in some instances, cannabis plants may also require an additional magnesium boost, especially if you administer calcium without magnesium.

How is Magnesium Deficiency Bad for Cannabis?

How is Magnesium Deficiency Bad for Cannabis?

If your plant does not receive enough magnesium during its growth, it can lead to various problems that can eventually kill your plant. 

The biggest danger is that your plant will be unable to produce carbohydrates and sugars (or energy) as it fails to photosynthesize properly. This can stop the plant from developing a strong root network and plentiful buds. 

In severe cases, magnesium deficiency can be fatal for cannabis. Without magnesium, it cannot produce enough energy to sustain itself, leading to wilting, followed by death. 

This can not only make your plant unhealthy, but even if the plant survives the deficiency until harvest, the buds will be of low quality and smaller in size. So, another danger is that magnesium deficiency can affect your plant’s yield. 

What Causes Magnesium Deficiency?

What Causes Magnesium Deficiency?

Given the importance of magnesium for healthy cannabis growth, you mustn’t overlook magnesium deficiency in your plant. But before you learn how to avoid the problem, you need to understand what causes it. So, here are some of the most common reasons why your plant may not be getting enough magnesium.

1. The Nutrient Solution Does Not Contain Enough Magnesium

The most obvious problem is that your nutrient solution or soil does not contain adequate amounts of magnesium. So, if your nutrients are incorrectly mixed, your plant may experience magnesium deficiency. Remember, it requires magnesium in higher quantities than most other micronutrients. 

2. You Overwater Your Cannabis

This may not only be because of incorrectly mixed nutrients, but also a side effect of overwatering. In some soils, magnesium can get easily washed away due to overwatering. Incorrect watering routine is perhaps the second most common reason why cannabis plants face magnesium deficiency. 

Fortunately, if you are using tap water, this likely won’t be the cause of your plant’s magnesium deficiency because tap water in most regions is rich in magnesium.

3. Low pH in the Grow Medium

pH is a small detail that can have a drastic impact on your plant’s growth, specifically, how it absorbs nutrients. So, even if the soil is rich in magnesium, if the pH level is too low (or acidic) for your plant, the roots may entirely fail to absorb the nutrient, leading to a deficiency. 

Here, even if you add more magnesium to the grow medium, it won’t solve the problem because the roots are not functioning efficiently. 

4. You’re Supplementing Calcium Without Magnesium

If you are supplementing calcium to your cannabis plant via a product that does not contain magnesium, like non-dolomite lime, your plant can experience magnesium deficiency. This is because high levels of calcium in the soil can hinder magnesium absorption, leading to a deficiency.

This is why CalMag supplements are so popular as opposed to individual calcium or magnesium supplements — both are best administered together to cannabis plants.

5. The Soil is Too Cool

Similar to low pH levels in the grow medium, low temperatures can also inhibit the roots from absorbing magnesium properly. If the soil’s temperature is under 60°F or 15°C, the plant may experience magnesium deficiency along with other deficiencies as the roots fail to absorb the nutrients. 

How to Identify Magnesium Deficiency in Cannabis?

How to Identify Magnesium Deficiency in Cannabis?

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Your cannabis plant can face many nutrient deficiencies, so how do you verify if it is a magnesium deficiency? It’s relatively straightforward. Magnesium is a mobile nutrient and the symptoms of its deficiency are somewhat unique and easy to identify. Follow the steps below to identify magnesium deficiency in your cannabis plants.

1. The Leaves are Losing Their Green Color

Magnesium is the primary element of chlorophyll — the compound that gives leaves their green color. So, with a lack of magnesium, your plant starts losing chlorophyll, and as a result, the leaves start losing their green color. 

So, if your plant is suffering from magnesium deficiency, one of the first signs you notice is the leaves fading in color or turning pale or yellow. This symptom is also known as chlorosis.

However, chlorosis is a common problem among various cannabis problems. What sets magnesium deficiency apart is the symptoms first show up around the leaf edges and between the veins. 

At the same time, often, you may also notice brown spots on the leaves if the problem gets worse. 

2. Older Leaves are Affected More Than the New Ones

Magnesium is a mobile nutrient, i.e., the plant moves it from old growth to new growth when required. So, signs of magnesium deficiency, especially chlorosis, tend to show up on the older leaves a lot earlier than they show up on newer leaves. 

The reason is, the plant withdraws this nutrient from older leaves and sends it to newer leaves to keep the new leaves alive, and in turn, keep itself alive. So, when you don’t address magnesium deficiency on time, the older leaves often start wilting and shedding first.

3. The Leaves Feel Crunchy

Cannabis leaves need to be soft and supple, but if they feel crispy or hard to touch, you are probably dealing with magnesium deficiency. The leaves feel like they may snap if you fold them in half. This symptom usually shows up a little late compared to leaf chlorosis.

4. The Buds are Smaller Than Expected

When a flowering cannabis plant suffers magnesium deficiency, it can be seen in the development of buds. For instance, the buds growing in a magnesium-deficit plant can be a lot smaller in size and have a much lower dry weight. 

Difference Between Magnesium and Calcium Deficiency

As mentioned earlier, magnesium deficiency’s symptoms are quite similar to other nutrient deficiencies, especially calcium deficiency. And since magnesium and calcium are closely linked to your plant’s growth, it is crucial to know whether your plant is suffering from magnesium or calcium deficiency. 

The first difference is that magnesium deficiency shows itself at the base of the cannabis plant in older leaves before getting onto the newer ones. On the other hand, calcium deficiency symptoms tend to show up on the top of the plant, newer leaves, first. During calcium deficiency, the new leaves may also wilt and lose color. 

The second difference is how the leaves lose their color. In magnesium deficiency, the leaves lose their green color and become pale or yellowish. On the contrary, calcium deficiency can turn the leaves yellowish and purplish. 

At the same time, a plant that is calcium deficient tends to have a low tolerance to heat — even minor heat fluctuations can affect the plant and exhibit signs of heat stress. 

Do note that both deficiencies can be fatal for your plant, but knowing the difference is crucial. Both nutrients affect how the other is absorbed and used by the plant, so if your plant is experiencing calcium deficiency and you mistake it for magnesium deficiency and compensate for the latter with more magnesium — the problem will quickly worsen and take your plant a lot closer to turning into mulch. 

How to Treat Magnesium Deficiency in Cannabis?

How to Treat Magnesium Deficiency in Cannabis?

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While magnesium deficiency can quickly turn fatal for your plant, it is still relatively straightforward to treat in cannabis. Yes, you can’t know the exact numbers — that would require expensive lab tests, but what you need to do instead is use the elimination method.

Start from the most common and probable cause of the deficiency and fix it before moving on to the next one. 

Follow these steps to treat magnesium deficiency in cannabis plants.

1. Allow the Soil to Dry Between Waterings

If the soil is too moist or if you overwater your plant, it may be flushing the medium of magnesium. So, the first thing you should do is correct your watering routine. Avoid watering your plant as often.

Ideally, you should water the plant early in the morning only when the top inch of the soil is dry. Bury a finger in the soil to check. In an indoor environment, you can even point a small fan toward the soil to keep it from becoming moist.

This will not only help you treat magnesium deficiency but also prevent root rot — a serious problem that affects the roots and eventually starves the plant.

2. Correct the pH Levels

If your watering routine is ideal, the next thing you need to do is check the pH levels of the growing medium. For this, it is recommended that you invest in a pH meter as it can help you test the pH of the medium accurately and easily. 

As mentioned above, pH can affect how your plant absorbs nutrients. The ideal pH range for the cannabis plant in soil falls between 6.0 to 7.0 and 5.5 to 6.5 for hydroponic plants. Use a pH meter to check the pH. 

If the pH is far off, this is likely the cause of magnesium deficiency. To correct it, you need to first flush the growing medium with pure water to remove any nutrient buildup followed by rectifying your nutrient solution. Ensure that your nutrient solution falls within the same recommended pH range. 

To rectify your nutrient solution’s pH range, you can use pH up or down solutions. They are readily available, cheap to buy, and effective at rebalancing the pH of the nutrient solution. 

Additionally, you should also check the pH of your tap water because it can vary, depending on the source and region. 

Next, check the pH of the runoff water — the runoff pH should be close to that of the nutrient solution, which indicates that there is no nutrient buildup within the growing medium. 

Some growers also prefer to keep the nutrient solution’s pH slightly basic. You can also do this. If you are growing in soil, you can keep the pH between 6.0 to 7.0, and for a hydroponic plant, you can keep it between 6.0 to 6.5 — magnesium tends to be absorbed better by roots above 6.0. 

3. Administer Magnesium Supplements to Your Plant

You can also administer some magnesium supplements to your plant to give it a boost. We recommend using CalMag solutions, but there are other alternatives to it, too. Here are the best magnesium supplements for cannabis.

Calcium Magnesium Supplements (CalMAg)

CalMag solutions are the most common treatments for either calcium or magnesium deficiency in cannabis plants. However, you need to ensure your plant is not suffering from a magnesium deficiency due to excess calcium in the grow medium.

Also, you may have to rebalance your nutrient solution with less calcium to maintain a balance between calcium and magnesium for your cannabis plant.

CalMag solutions are easy to find in local gardening stores and are easy to use. Generally, you will have to mix a teaspoon of this solution in a gallon of water but always refer to the manufacturer’s recommendations for the same. Some solutions may have entirely different formulas.

  • Organic Dolomite Lime 

A great alternative to CalMag solution is the organic dolomite line — a powder that is rich in calcium as well as magnesium and can be easily mixed in with the soil. Plus, you only have to apply it once as it releases the nutrients slowly over the next few months.

Another benefit of dolomite lime is that it has a neutral pH (7.0) so it won't affect the pH drastically. 

When purchasing dolomite lime, always go for the finest quality you can find — it will act faster than granular dolomite lime varieties. You can purchase high-quality dolomite lime at your local gardening store for a cheap price. 

And to use it, add around 6 to 7 teaspoons of it to each gallon’s worth of soil. Then, mix it well with the soil followed by light watering. After watering it, mix the soil once again to ensure even distribution of dolomite lime in the root zone. 

If you are growing your cannabis outdoors, read the manufacturer’s recommendations on using dolomite lime as it can vary from brand to brand. 

  • Epsom Salt

If you have just noticed magnesium deficiency in your plant but it’s the weekend and the local gardening store is on the other end of the city, you can use Epsom salt — a terrific emergency fix for magnesium deficiency in plants. 

Epsom salt, scientifically known as magnesium sulfate, can be purchased from any pharmacy (or even grocery stores) and is a rich source of magnesium. It is, after all, mostly made of magnesium. 

And since Epsom salt is easily soluble in water, you can even use it as a foliar spray to treat the problem faster. Simply mix a tablespoon of Epsom salt with four liters of water, and spray it onto your plant’s canopy every three days. Also, spray the canopy down with clean water every ten days to avoid buildup on the leaves. 

The best time to spray some Epsom salt on cannabis plants is during the dark hours when the temperatures are low — this is when the plant can absorb the nutrient efficiently. 

If you are growing cannabis in a greenhouse, you need to be conservative with foliar spray as excess humidity in the greenhouse can attract mold and fungi. And avoid spraying the buds at all costs as Epsom salt can leave a nasty aftertaste on the buds. 

  • Increase Temperature at the Root Zone

Cannabis favors a slightly warmer climate, so temperatures below 60 degrees Fahrenheit can cause magnesium deficiency. So, if the root zone is less than 60°F(15°C), you need to warm it up.

You can warm up the grow medium by using a heater in an indoor environment; an electric blanket can also work in a pinch. For outdoor plants, you can move the plants indoors during cold nights or place a tarp on the soil during the night. And if you are feeling a little adventurous, you can even set up a greenhouse around your plant to keep it warm and cozy. 

Magnesium Deficiency Recovery Time

Once you have corrected the problem, you need to wait for a couple of days before the problem stops getting worse, i.e., for the symptoms of leaf chlorosis from growing. However, do note that the affected leaves will not recover or regain their original color. 

If the problem was at the root zone, the problem might even improve immediately. 

During the recovery time, you need to pay close attention to your plant and don’t prune the far-affected leaves just yet. Wait until you are sure the problem is solved. Old leaves may still be a source of magnesium for your plant, which is why you don’t want to prune them immediately. 

How to Prevent Magnesium Deficiency in Cannabis?

How to Prevent Magnesium Deficiency in Cannabis?

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Magnesium deficiency is even simpler to avoid, so use these tips to prevent magnesium deficiency from ever occurring on your cannabis plants:

  • Ensure the soil or grow medium is nutrient-rich and contains the right balance of both calcium and magnesium
  • Make sure the nutrient solution contains an adequate amount of magnesium for a healthy growth 
  • If you notice a deficiency developing, you should consider flushing the plant and rebalancing the pH levels 
  • Keep the root zone warm, above 60°F

With these basic steps, you can almost always prevent a magnesium deficiency in your cannabis plant. 

Summary: How to Fix Magnesium Deficiency in Cannabis Plants?

Magnesium is crucial for your cannabis plant and its deficiency is easy to spot if you look closely enough at the leaves. However, the more you learn about the treatment for magnesium deficiency in cannabis, the more everything looks simple. 

As you can see, treating magnesium deficiency is largely about taking care of the minor stuff that we growers often miss and less about investing a lot more time and energy into the process. 

In fact, you can even use a small packet of Epsom salt to quick-fix magnesium deficiency — an example of how trivial the treatments for some hardy problems can be. But magnesium deficiency is only one of the many that you might face in your growing career. 

If you want to know about other serious problems your plant can face and how you can fix them the smart and easy way, stay tuned. Remember, growing cannabis is about working smart, not working hard. Although, hard work does pay off with bigger buds. 


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Great Article thank, having this at the moment so very handy 😊
Thank you I needed this!