Magnesium Deficiency In Cannabis

You will be able to identify a cannabis magnesium deficiency if your plant shows yellow spots on the edges or between the veins of its older leaves, while the veins themselves remain dark green.

Magnesium is a so-called ‘secondary’ nutrient and is required in large quantities at all stages of marijuana, both in vegetative and flowering stages. Fortunately, this cannabis deficiency is easy to solve.

Colors and Symptoms Of Magnesium Deficiency

magnesium-deficiency-color-chartMagnesium deficiency symptoms will begin on the older, lower leaves, which will show a yellowing between their dark green veins.

In cannabis, it begins at the tips of the leaves moving towards the base (unlike an iron deficiency, which begins at the base).

The edges of these leaves can also become rough to the touch, and will turn yellow or lighter green. 

The yellowing will begin to rise to higher leaves as the deficiency worsens.

New shoots will grow pale green, and the stems and petioles will turn purple.

The most deficient leaves will begin to fold on themselves and will quickly die. In some marijuana plants you will see that the leaves have small brown spots where the tissue has died.

Leaf ColorSymptoms
They start with yellow spots between veinsLeaf tips begin to die
Edges start yellowing as wellLeaves curl on themselves (upwards)
Veins stay greenThe plant loses strength and growth slows down
Brown dots appear on the leavesMost affected leaves quickly begin to die

If this don't seem like your plants, check other common cannabis leaf symptoms with pictures and how-to guides.

The Importance Of Magnesium

Magnesium is a central component on chlorophyll, playing a key role in the photosynthesis process that cannabis, and all plants, need to thrive.

It also helps to carry phosphorus and with phosphate metabolism, which is of upmost importance during flowering stages.

As well as many other reactions and functions without which marijuana plants would simply die.

Solve Root Problems To Allow For Magnesium Absorption

Generally, in cases of magnesium deficiencies, the nutrient is present in the soil in sufficient quantities. However, if  there are problems in the plant root system, your marijuana plant will not be able to properly absorb magnesium.

Since root problems are the most common causes of magnesium deficiency it is also possible that along with these symptoms you will see others that respond to deficiencies of other nutrients.

Adding magnesium in this case will not help you to solve the deficiency, and can even generate more problems and compromise your plant long term.

pH Too Low For Cannabis

ph-meter-exampleChecking the pH of the root zone is the first step before trying another solution for a cannabis magnesium deficiency.

If the pH of the growth medium is too low (very acidic), magnesium can't be absorbed by the roots.

In soil, the roots will better absorb magnesium if the pH is within the 6.0-7.0 range. Below 6.0 you will start to see problems, and if it drops to 5.0 it will generate a complete crash.

You will need to increase the pH level, as you may have already guessed 😆

Read below about how dolomite lime will help you neutralize the pH of the soil and add good amounts of magnesium.

Constant Moisture In The Soil

If you notice that your soil is constantly wet, then this may be the cause of magnesium blockage.

Fixing it is not only important for magnesium absorption, but you will also be avoiding possible root rot, which would be fatal for your marijuana plant.

The first thing you should do is take care not to water your plants as often; allow the soil to dry between waterings.

As a general rule you can bury your finger about 1 inch (2 centimetres), and if you find it dry, then you should water.

If you are growing your plants indoors, a quick fix can be to place a fan pointing directly at the soil; moving the air will make the task easier.

Low Root Temperatures Can Make It Magnesium Deficient

Marijuana, regardless of its variety, favors a warm climate to grow properly. Below 60ºF (16ºC) the roots will be unable to function properly.

If you’re growing indoors, try to install a heater for grow tents. An electric blanket on a timer is one good option.

If you have outdoor pots, you can bring them into the house during cold nights.

If you can’t move the plants, you will need to cover them in some way. It can be simply with a tarp to prevent night dew, or you can even take the trouble to raise a greenhouse around it.

Coming soon is a guide on how to build a cheap and efficient greenhouse.

Mineral Excess During Flowering

You may see a magnesium deficiency during the flowering period if you have been adding flowering fertilizer.

An excess of minerals (from the abundant nutrients in flowering fertilizer) can prevent your plant from absorbing magnesium properly.

To fix it simple flush the roots, and continue with recommended fertilizer for your current stage.

Using Magnesium Supplements

You need to supply magnesium to your plant if you are sure that you have the correct pH, that you are not overfertilizing, and that you are taking care of the roots of your plant with correct watering frequencies and temperatures.

I reiterate that adding magnesium in a blocking situation will not help and may even worsen the situation. Review the previous sections if you’re unsure what to do.

Cal-Mag Supplement

Magnesium deficiencies are often accompanied by calcium deficiencies. If this were the case, you can easily fix it with a calcium and magnesium supplement (there is one called Cal-Mag but there are many similar ones).

Epsom Salts For Magnesium and Sulfur


Magnesium sulfate is an inorganic salt that is commonly marketed as “Epsom salts.”

These salts are soluble in water and efficiently supply magnesium to your plants, either by mixing it in the soil or by foliar application (spraying the solution on the leaves).

In gardening and agriculture, Epsom salts are used to correct magnesium or sulfur deficiencies.

The advantage of these salts over other magnesium additives such as dolomite is their high solubility, which also opens up the possibility of foliar feeding.

How To Do Foliar Feeding:

  1. Heat the water to further improve the solubility,
  2. Then you add 1 teaspoon for every 1 liter of water,
  3. Wait until the water returns to room temperature,
  4. Spray the leaves liberally with this solution, just enough to cover them completely (also underneath), but they don’t need to be dripping with water.

Be very careful with foliar feeding if you're using a grow tent, as the extra humidity can attract fungi and ruin your crop. And also pay important attention not to wet the buds in any case.

Wait until the dark hours before spraying your plants. Low temperatures and darkness favor absorption, regardless of whether you grow indoors or outdoors. In addition, water droplets can generate a magnifying effect that will cause light or rays of the sun to burn the leaves.

Providing magnesium in this way will ensure a quick recovery of your marijuana plants.

Dolomite Lime For Pot Or Soil


If you are looking for a way to add magnesium to your substrate, a recommended option is dolomite lime, also called dolomite.

You can buy dolomite in nurseries and garden stores. It is a great source of nutrients that combines calcium and magnesium, and is commonly used in commercial crops.

Dolomite has a neutral pH of around 7.0 and will help buffer the pH variations in the soil and keep it in an optimal range for marijuana growth, especially on acidic substrates.

How To Use Dolomite Lime:

Apply dolomite lime to your plants by sprinkling it on the substrate, so it is absorbed very slowly. This method can be positive or negative:

Positive because it will continue to provide nutrients over the following months. Negative in case you have magnesium urgency.

  • If you are growing in pots, sprinkle about 6 tablespoons per 4 liters of substrate above the ground.
  • If you are cultivating directly in the soil, take a handful and sprinkle over the soil around the plant.

Then water with standing water slightly, just to moisten the lime and the soil. The next day you can water normally.

Have patience. You will need to wait at least one week to start seeing changes in the pH of the soil. The finer the dolomite the faster its nutrients will be released.

How Will You Know The Deficiency Has Ceased?

Once you have corrected the problem or problems, after a few days the yellowing will stop increasing in the affected leaves and stop spreading to other leaves. You will see the growth resume its normal rhythm and new shoots will be healthy and green.

However, damaged leaves will probably not revert to their original color.

It is important that you pay special attention and do not remove the affected leaves until you are sure that the deficiency has ceased. If the problem has not been solved, the plant will continue removing magnesium from the leaves, and if you remove the affected leaves, the discoloration will appear in other leaves that until now were healthy.

Thanks For Reading

As with other secondary nutrient deficiencies, the problem is usually in the roots. Especially if you have used a prepared soil, fertilize normally and water correctly.

There are many people who think that marijuana does not need so much care, that measuring pH is not necessary, or that marijuana is a weed and grows anywhere.

While it is true that cannabis is generally a very hardy plant, or a “weed,” our goal is not for it to simply survive, but to develop to its full potential to yield the largest possible harvests and good THC quantities. And that is why it’s important to take good care of your plants, and above all take good care of your root system.

I hope this guide has helped you, I will continue uploading guides throughout the year. If you want to be notified when I publish a new one, you can sign up to the mailing list below. I promise to send you only quality content.

Hey there, I'm the person behind this website. I started growing cannabis in 2014 right after marijuana was legalized in my home country. Today, on, I address growers need for knowledge and show you how to grow cannabis plants and enjoy amazing buds.

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