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Magnesium Deficiency and Toxicity in Cannabis Plants

by | Aug 8, 2022 | Getting Started, Nutrients, Troubleshooting

Magnesium is a mineral that is an essential plant nutrient to help them to grow and stay healthy. It is needed for photosynthesis, which means magnesium deficiency can lead to stunted growth in cannabis plants. In this article, we explore magnesium deficiency and toxicity in cannabis plants. We also provide tips on preventing magnesium deficiencies and toxicities so you can maintain your plant’s health.

What is magnesium?

Magnesium is a mineral that aids in the absorption of calcium. It is essential for the plant’s process of photosynthesis. Magnesium can be found in the soil, but when magnesium levels become too low or too high in the plant’s roots, it can cause magnesium deficiencies or toxicities. These problems are very common for new growers and thankfully can be fixed very easily.

Why do cannabis plants need magnesium?

Magnesium has an important role in photosynthesis and for using energy produced by respiration to break down sugars into ATP, or Adenosine 5′-triphosphate, which is used to store energy in cells. This energy can then be used as fuel for cellular functions throughout magnesium’s interactions with the chlorophyll molecule during photosynthesis as magnesium acts as a cofactor, meaning a substance that needs to be present for an enzyme to perform certain actions, to many enzymes involved in photosynthesis. Therefore, it is needed at all stages of cannabis growth including the vegetative phase and flowering phase.

Magnesium is also necessary for the synthesis of RNA, DNA, and protein in plants. It plays a role in enzyme activity by stabilizing magnesium enzymes that catalyze reactions throughout different biological processes including glycolysis, photophosphorylation, conversion of ATP to ADP or AMP (adenosine monophosphate), and the production of alkaloids, terpenes, tocopherols (vitamin E) in cannabis.

Magnesium is also necessary for magnesium-specific enzyme activity involved in carbohydrate metabolism which includes fatty acid synthesis by converting acetyl CoA into malonyl CoA or FAs (fatty acids). Magnesium also works with magnesium-specific enzymes involved in protein synthesis, including the production of melanin or a pigment that protects cannabis from UV radiation.

Is magnesium a macronutrient or micronutrient?

Magnesium is a micronutrient. Micronutrients are minerals needed by plants in small amounts for healthy growth. On the other hand, macronutrients are nutrients the plant needs in larger amounts. The three macronutrients for cannabis are NPK, or nitrogen, phosphorus, and potassium.

Is magnesium a mobile or immobile nutrient?

Magnesium for plants is a mobile nutrient because it moves freely from the soil into the plant’s cells. Mobile nutrients are nutrients that can move from one part of the plant to another, while immobile nutrients are nutrients that remain in a certain part of the plant and can’t be moved to other parts. Mobile nutrients include magnesium, calcium, sulfur, and potassium while immobile nutrients include calcium and iron. A good rule of thumb to discern between mobile and immobile nutrients is seeing where the deficiency manifests itself on the plant. If the deficiency is in the older leaves, typically this is a mobile nutrient as it will move the nutrients it needs from the older leaves to the newer ones. If the deficiency shows up in the newer leaves, this is probably due to an immobile nutrient as the plant doesn’t have enough of that nutrient and can not move it from other parts of the plant.

What are the symptoms of magnesium deficiency in cannabis plants?

One of the most common magnesium deficiency symptoms is interveinal chlorosis, which causes green leaves to change color from their normal green hues to yellow or white due to a lack of magnesium present in chlorophyll while the veins stay green. Other common visual symptoms include mottle leaf tissue along with necrosis or the death of tissues, brown spots, or lesions on leaves resulting in necrotic tissue that turns brown. Young leaves may curl around the edges and magnesium deficiency can also cause stunted growth throughout magnesium’s role as a cofactor for many enzymes involved in photosynthesis. In the advanced stages, the leaves with the most damage will brown, curl up, and die. Magnesium deficiencies will start with the older leaves and move their way up the plant to the newer leaves over time. The deficiencies will start at the leaf tips and work their way down the leaf toward the stem.

How do I fix a magnesium deficiency in my cannabis plants?

The easiest way to fix a magnesium deficiency is to use a calcium and magnesium nutrient supplement, also called CalMag, that is readily available at garden centers. Magnesium deficiencies can be fixed with magnesium sulfate, which is found in both CalMag and Epsom salts too, or magnesium chloride. Magnesium sulfate is typically used more often than magnesium chloride as magnesium chloride needs to dissolve in hot water, while magnesium sulfate dissolves well in cold water which is easier for most gardeners.
How do I prevent a magnesium deficiency in my cannabis plants?
Prevent magnesium deficiencies by providing magnesium to your roots. Magnesium is readily available in the soil, but if you are growing hydroponically or using grow mediums that do not have magnesium present it’s important to supplement with magnesium through a CalMag fertilizer.

What is magnesium toxicity in cannabis plants?

Magnesium toxicity is very rare for cannabis growers, especially if you are using soil that was made for growing marijuana plants. Magnesium toxicity is typically caused by over-fertilizing magnesium. The symptoms of magnesium toxicity are very hard to diagnose for a couple of reasons. First, it is very rare so isn’t encountered often and growers will usually attribute it to something else. Second, magnesium toxicity will lead to a calcium lockout which will cause a calcium deficiency and will therefore look like a calcium deficiency. One of the symptoms of a magnesium deficiency includes the leaves turning dark brown or black.

How do I fix magnesium toxicity in my cannabis plants?

Due to excess magnesium causing a lockout of calcium, the best way to fix toxicity is to flush your plant. To do this, you simply run three times the normal amount of PH’d water through your plant medium. This should clear up any lockouts and wash away the excess magnesium. If the toxicity is due to using the wrong soil, we recommend changing your substrate to something more amenable to cannabis plants.

How do I prevent magnesium toxicity in my cannabis plants?

You can prevent magnesium toxicity by using proper soil, if growing in soil, and making sure your water with nutrients falls in the proper PH range for cannabis to uptake nutrients from your medium. This will help prevent a nutrient lockout, which becomes a problem with magnesium toxicity due to it locking out calcium.


Magnesium is an element that is necessary for photosynthesis in plants among several other important processes. It is a micronutrient that is mobile, which means that is needed in smaller quantities than NPK and is able to move from one part of the plant to the other.

There are several symptoms of a magnesium deficiency, but usually one of the first and most common symptoms is interveinal chlorosis. This means that your plant is yellowing in between the veins of the leaf while the veins stay green. Eventually, the leaves will brown, show dark, necrotic spots, shrivel up, and die if left untreated. To fix a magnesium deficiency, simply add a readily available calcium and magnesium nutrient solution that is abbreviated as CalMag.

To prevent a deficiency, use CalMag fertilizers in your soil. Magnesium toxicity is very rare and will look like other nutrient deficiencies, especially calcium deficiency due to it locking out calcium from the root system. Dark brown or almost black leaves can also appear with magnesium toxicity. You will need to flush the plant with three times the normal amount of PH’d water to flush out any excess magnesium and allow the roots to start taking in calcium again.

Preventing magnesium toxicity is easy, all you have to do is use a medium that has been created for cannabis and not overdo it with the CalMag nutrients. Should you have any further questions, please see our articles regarding calcium deficiency, flushing your plants, and how to PH your water.

To learn more about growing cannabis at home consider checking out our guides on: Getting Started, Troubleshooting, and Optimizing your Grow or search our Knowledge Base.

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