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Magnesium Deficiency in Cannabis

Cannabis leaves, cannabis on a dark background, Mature Cannabis

Magnesium deficiency in cannabis is among the most common nutrient issues. Magnesium is commonly linked to another micronutrient, Calcium, a deficiency of one will likely lead to a lack of both, this is called cal-mag deficiency.

What does magnesium do for cannabis plants?

Magnesium is a necessary component for the growth and overall health of your cannabis plants. Magnesium is often found in the plant’s leaves as it is essential for chlorophyll molecules. As a result, its presence or absence directly affects the plant’s capacity to absorb light and, thus, produce sugars and carbohydrates. Your plants will struggle to grow if their magnesium levels are low. Additionally, magnesium is the most important secondary mineral for the healthy growth and development of cannabis plants after the three primary nutrients (nitrogen, potassium, and phosphorus.) Magnesium is a mobile nutrient that moves throughout the plant. This indicates that the cannabis plant can move it from older to newer tissue. This is why symptoms of magnesium deficiency in cannabis plants appear first on the older, lower leaves rather than on new growth.

Similarly, magnesium fits well when paired with calcium, ensuring that your cannabis plants receive adequate amounts of essential minerals. Cal-Mag deficiency is the de-facto name for a lack of calcium and magnesium.

How to identify magnesium deficiency in cannabis

Magnesium is the basic ‘cornerstone’ of chlorophyll, which gives the plant its vibrant green color. The first and most obvious indicator of magnesium deficiency in cannabis plants is a troubling fade of green to pale green to yellow on the leaves. Cannabis magnesium deficiency first shows on the edges and between the veins of the leaves, rather than all over or from the stems outwards or tips inwards. The oldest and closest leaves at the bottom of the plant will be the first to show these signs. This is due to the plant removing magnesium from these leaves to transport it to the newer ones and stay alive. We recommend you start treating magnesium deficiency when you first spot it, do not wait until the leaves begin to turn dry and crispy.

Common symptoms of magnesium deficiency

Magnesium deficiency usually starts on the older leaves later in the vegetative growth cycle or during bud formation. The following are the significant signs and symptoms of a magnesium deficiency in cannabis:

  1. Your cannabis plants show yellow spots on the edges or between the veins of the older leaves, while the veins are dark green.
  2. Purple coloration will appear on the stem and petioles.
  3. Stunted lower leaf growth.
  4. In advance stages, interveinal chlorosis spreads inwards to the center of the leaves, eventually becoming dry and crispy.  


Dark green leaves indicate a healthy pot plant. Plants with a magnesium deficiency have fading green or yellow leaves. These signals appear first on the biggest leaves near the bottom. Magnesium transfers from larger leaves to younger, smaller leaves. The huge leaves will keep doing this until they run out of magnesium. Leaf discoloration occurs as a result of a magnesium deficit in cannabis.


Micronutrient deficiencies, including magnesium deficiency, always begin in the root zone. You must address the issue at its root. The roots of the cannabis plant drink up all of the micronutrients in the medium as it matures. A cannabis root system delivers nutrients to the leaves through the healthy, dark green stem. Any missing components will jeopardize plant function. The stems of cannabis plants turn reddish-purple when they are deficient in magnesium.

Abnormal plant growth

One of the signs of cannabis magnesium deficiency is abnormal growth. With magnesium deficiency, the leaves will start to have abnormal growth. It can impair the cannabis plant’s health and lower yields if left unchecked. 

What causes magnesium deficiency in cannabis plants?

When growing cannabis plants hydroponically or cultivating cannabis indoors in any medium, magnesium deficiency is common. Magnesium deficiency is common in hydroponic cultivators who allow the pH of the nutrient solution to fall below 5.0. Water with a pH that is too low in the reservoir leads to micronutrient lockout. Cannabis magnesium deficiency is common among indoor soil gardeners for two reasons. The first is that the soil is over-watered. Roots in wet soil cannot get to the nutrients.

Growing cannabis indoors
Magnesium deficiency happens when growing cannabis indoors if there is too much water

The second, on the other hand, is more difficult to discern. Growers who employ minimally fertilized soil and trace-element-deficient liquid nutrients may experience a magnesium deficit during mid-late bloom. The roots of the cannabis plant drink up all of the micronutrients in the medium as it matures. In the absence of sufficient magnesium, the deficiency strikes!

How do you solve cannabis magnesium deficiency?

Since the symptoms of magnesium deficiency can be mistaken for those of other deficiencies, additional investigation of leaf tissue is required to identify the correct deficiency. Magnesium deficiency can be caused by a lack of nutrients, uneven soil pH, or imbalanced fertility, leading to antagonism from other nutrients such as too much potassium, calcium, or sodium.

You can correct magnesium deficiency by adding or feeding Epsom salts, magnesium sulfate, and other magnesium-containing, water-soluble nutrients to your cannabis plants. Use worm castings, dolomitic lime, or garden lime for an average degree of magnesium absorption. Dolomitic lime is used to correct magnesium deficiency and raise the pH of the soil medium and other planting combinations. You can use dolomitic limestone to treat magnesium deficiency and raise soil pH because it has significant magnesium. Cal-Mg can also be used to speed up the absorption process.

Solving magnesium deficiency is quite similar to treating other cannabis deficiencies, however, it’s best to research them all to have a good working knowledge.

Plant problems with similar symptoms to magnesium deficiency

Iron deficiency is comparable to magnesium deficiency, but the newest leaves will yellow and suffer from an iron deficiency. When young leaves turn yellow, it indicates an iron shortage. Also, calcium deficiency is similar to magnesium deficiency due to the yellowing of the leaves that appear in the bottom of the cannabis plant at first. However, the color will remain in calcium deficiency, but it will appear in a magnesium deficiency throughout the crop.

As mentioned previously, magnesium deficiency often comes combined with calcium deficiency – this is referred to as a cal-mag deficiency. Learn how to solve calcium deficiency in cannabis plants to help avoid a cal-mag issue.

Final word on magnesium deficiency

If you have a magnesium deficiency in your cannabis plants, you will notice the symptoms. You will see yellowing starting at the lower leaves and red stems. To remedy the problem, flush your plants and upgrade your fertilizers or add supplements. It is always preferable to prevent than cure. Keep an eye on the pH balance of your growing medium and water, and do not overwater your cannabis plants. Also, you can use a well-balanced fertilizer.

Now that you know what a magnesium deficiency in cannabis plants is, you can watch your crops. Purchase high-quality cannabis seeds from Premium Cultivars and begin growing right away. To avoid and treat cannabis magnesium deficiency in cannabis, take care of your plants and know the indicators.

Picture of Ed Rushford

Ed Rushford

Ed Rushford’s impact on cannabis growing is undeniable. Though he tends to focus primarily on 2 areas, plant training techniques and dealing with disease, pests, and other problems, he has offered many insights into how cannabis plants live and grow. That’s not to say that Ed is unfamiliar with the complete life cycle of cannabis, from seed to harvest, but he uses his widespread knowledge to hone in on the minutia and niche areas of growing cannabis. Ed’s goal is to spread knowledge and allow for everyone to become better growers. About this Author

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