Magnesium Deficiency in Marijuana

You will be able to identify a 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.

chart magnesium deficiency in marijuana
Chart by Jorge Cervantes

As it progresses it will also affect the leaves of the middle part. The leaves may show spots of necrosis (brown spots), and the most affected ones will fold over themselves and die.

Magnesium is a so-called ‘secondary’ nutrient and is required in large quantities at all stages of marijuana, both in growing and flowering stages.

As magnesium is a ‘mobile’ element, when the plant needs it, it can remove it from the less important leaves (the old leaves) and take it to the new and more important leaves.

Fortunately, this deficiency is easy to solve.

Colors and Symptoms

In Lower / Older Leaves

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 leavesThe most affected leaves quickly begin to die

Symptoms will begin on the older, lower leaves, which will show a yellowing between your dark green veins.

Magnesium deficiency begins at the tips of the leaves moving towards the base, unlike iron deficiency that begins at the base.

The edges of these leaves can also become rough to the touch, turning yellow or lighter green. It is difficult to get confused, but it is necessary to clarify for the novice eye in the cultivation of marijuana that this situation is different from the excess of fertilizers which first burns the tips of the leaves.

The yellowing will begin to rise to higher leaves as the situation worsens, the new shoots will grow pale green, the stems and petioles will turn purple.

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

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

Magnesium Deficiency Pictures

Solve because of Root Problems

Low pH

Checking the pH of the root zone is the first step before trying another solution.

Generally, in cases of magnesium deficiencies, the nutrient is present in the substrate in sufficient quantities. However, if the pH of the growth medium is too low (very acidic), your marijuana plant will not be able to properly absorb magnesium.

Adding magnesium in this case will not help you to solve the deficiency and can even generate more problems and blockages of other important nutrients.

In substrate, 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: P

A simple technique in this case is to rinse the soil with pH regulated water and a low dose of marijuana nutrients that includes magnesium. Remember, first of all, let the water stand in an open container for a minimum of 24 hours.

You have a complete article on dolomite lime that will help you neutralize the pH of the substrate and good amounts of magnesium.

This will cleanse the root area of any excess salts that may be causing the pH variation and blocking the absorption of magnesium.

You can also see our pH guide with other methods of varying the pH of your soil.

Constant Moisture in 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 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. Allows the soil to dry between watering and watering.

As a general rule you can bury the finger about 1 in (2 cm) and if you find it dry, then you water. 

Coming soon: Marijuana watering guide.

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

Low Root Temperatures

Marijuana, regardless of its variety, favors a warm climate to grow properly. Below 18ºC the roots will have problems to function properly.

Indoors try to install a heat source. A timer electric blanket is a good option.

If you have outdoor pots you can directly enter them into the house during cold nights.

In case 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: Guide on how to build a cheap and efficient greenhouse.

During Flowering – Excess Potassium

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

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

To fix it we rinse the roots in the same way that I mentioned in the “Low pH” section.

Solve by adding Magnesium

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

I reiterate, for the doubts you have not read, that adding magnesium in a blocking situation will not help and may even worsen the situation. Review the previous sections if you haven’t read them yet

Back to the topic … these are your options:

Magnesium supplements

Epsom Salts

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).

Magnesium sulfate is an inorganic salt that is commonly marketed as “Epsom salts”. These salts are soluble in water and very efficient to supply magnesium, either by mixing it in the substrate 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 application.

Foliar Application of Epsom Salts

  • First of all you should heat the water to further improve the solubility.
  • Then you add 1 teaspoon for every 1 liter of water.
  • Spray the leaves liberally with this solution, just enough to cover them completely (also underneath), but they don’t need to be dripped with water.

It is important not to wet the buds, the extra humidity can attract fungi and ruin your crop.

Regardless of whether you grow indoors or outdoors, wait until the dark hours before spraying your plants. Low temperatures and darkness favor absorption. 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

Dolomite Lime

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 it 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

It is applied by sprinkling on the substrate, so it is absorbed very slowly. This can be positive or negative.

Positive because it will continue to provide nutrients over the 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.
  • In case 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.

Patience. You will need to wait at least 1 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 also spread to other leaves. You will see the growth resume its rhythm and the new shoots will be healthy and green.

However, damaged sheets 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. Because 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 substrate, 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, 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 that’s why it’s important to take the trouble to take good care of yourself, and above all take care of your root system.

I hope this guide has helped you, I will continue uploading guides throughout the year, if you want to find out you can sign up to the mailing list below. I promise to send you only quality content.

Santiago @ Dr. Cannabis
Santiago @ Dr. Cannabis
I'm here to help out marijuana growers have a greater experience with their plants, and not loose their mind over the many problems and situations that often arise during the different stages of cannabis growth.
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