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Magnesium deficiency weed

Magnesium Deficiency in Cannabis: How to Identify and Fix It

Are your plants looking a little strange? If you suspect a magnesium deficiency in cannabis, put your detective eyeglass away. We’ve got everything you need to know about a lack of this crucial micronutrient. 

Read on as we break down the causes, signs, and fixes for a magnesium deficiency. We also tell you about rare cases of nutrient toxicity and how to spot it.

Let’s get started.

Causes of a magnesium deficiency in weed

Learning about the causes of a magnesium deficiency in weed helps prevent the problem. This shortage occurs for several reasons, including incorrect pH levels, plant stress, lack of nutrients, and overwatering.

If the pH levels are off balance, some issues may arise. A magnesium (Mg) deficiency typically happens when you don’t achieve the best pH for cannabis. It’s more prevalent with hydroponic growing mediums since the levels tend to be lower.

As a result, the roots struggle to absorb Mg in the soil or water, so adding supplements doesn’t help. It causes a nutrient lockout where the medium holds the nutrition, but the roots can’t carry it throughout the structure.

When stressed, plants struggle to absorb nutrients like magnesium, resulting in drooping leaves and reduced harvest. Common stressors include:

  • Overwatering
  • Too little or too much light
  • Extreme temperature changes
  • Cannabis nutrient burn
  • Changing location

If you grow outdoors, the native soil may lack essential micronutrients, leading to a magnesium shortage. Flushing the flora without replenishing the nutrients could also lead to a deficiency. 

Overwatering drains the nutrients from the soil, neutralizes the pH, and results in a lockout. This action causes plants to get soggy roots and damages their essential structure with time. 

Identifying a magnesium deficiency

Spotting a magnesium deficiency is a lot easier than with other nutrient shortages. Here are the signs to look out for:

the signs of cannabis magnesium deficiency
The symptoms of cannabis magnesium deficiency

Yellow leaves

Healthy marijuana plants typically have dark green foliage. One of the magnesium deficiency symptoms in cannabis is yellow leaves. At first, the top shrubs still appear fresh, and growth is unaffected while damage occurs at the bottom. 

yellow bottom leaves
Yellow bottom leaves can be the major sign of magnesium deficiency

Why does only the lower foliage fade? Mg is a mobile nutrient that travels from aged leaves to new ones. Older foliage taps into mineral storage when a shortage occurs and sends it to the younger magnesium deficiency leaves.

Yellowing between leaf veins indicates a lack of mobile nutrients and reveals that the foliage is transferring the necessary minerals.

It also signals other mobile nutrient shortages, like a nitrogen or calcium deficiency in weed. To confirm a lack of Mg, look for a yellow rim around the edge of your plants.

Red stems

Chlorophyll is a pigment responsible for helping flora absorb light, making the foliage green. The importance of magnesium contributes to its production and movement. When lacking, plants struggle to absorb light and produce energy from photosynthesis.

red and brown spots on leaf foliage
Red and brown spots on leaves is a sign that your plant is suffering from magnesium deficiency

Some parts start turning a reddish-purple color as the chlorophyll levels decline. This change typically begins at the base, affecting the roots, lower colas, and bottom foliage. 

purple sponts on cannabis leaves
When your plant lacks magnesium, you can see purple spots on the leaves. like these ones at the picture above

If you spot red stems and suspect a magnesium deficiency in marijuana, it’s time to act fast. 

a purple color appears between the veins
Puple color between the veins should raise a red flag

Once the stems darken, your greenery struggles to deliver nutrients to the upper leaves. If left untreated, the dangers of magnesium deficiency include necrosis, and your cannabis crops may eventually starve to death.

Sometimes, you might cultivate high-quality weed seeds from a strain that organically grows red. Plum stems are a natural formation, so if your plant looks healthy, it’s probably a purple variant. 

purple cannabis stem
Purple stem is another symptom of magnesium deficiency. In most cases, it can be caused by genetics. If you are sure that it is not genetics, it may be a symptom of the lack of Mg

Changes throughout the plant

As a mobile nutrient, a lack of magnesium causes many changes throughout the plant. Other mineral issues, including a Cal-Mag deficiency in cannabis or a shortage of immobile nutrients like calcium, affect only one area.

The changes you see depend on the stage of the deficiency. Here are some signs to look out for in the early phases:

  • Lower and middle fan leaves turn yellow
  • Older leaves have crisp and brittle edges
  • Margins turn yellow while interveinal space remains green
  • Reddish or purple stems
interveinal cholorisis
A major sign of the lack of magnesium is when margins turn yellow while interveinal space remains green

The symptoms worsen if you don’t solve the problem. Here are some red flags that reveal a late-stage magnesium deficiency:

  • Brown spotting within the leaf margins
  • Necrosis of bottom to mid foliage
  • Necrotic spots or markings on new leaves
necrotic spots on leaves
As magnesium deficiency progresses, you may see necrotic spots on the leaves
small white spots on weed leaves as a result of magnesium deficiency
Small white spots can also identify magnesium deficiency, but very rarely

How to fix a magnesium deficiency in cannabis

Fixing a magnesium deficiency in weed plants is usually straightforward, depending on how far it’s progressed. In most cases, flushing and supplementing get the job done.

A fresh start works wonders for most nutrient deficiencies in cannabis plants. Follow these steps to flush your flora:

  1. Water your greenery thoroughly using 6.0 pH water until all the substrate washes out. 
  2. Add a balanced fertilizer that won’t offset the pH of your soil to your grow medium. 
  3. Give your plants an extra magnesium supplement by mixing it with the fertilizer to counteract the low Mg levels.
  4. Depending on the severity, you should see results in 2–6 weeks. Positive signs include new growth and old leaves falling off.

It’s essential to consider your growing medium when treating mag deficiency weed. Here’s how to fix it in each substrate:


Use 6.0–6.5 pH water. If the level is below 6.0, it’s too acidic. Flush it out and replenish it with nutrients within the optimal pH range.

It’s a good idea to use a foliar spray to treat a Cal-Mag deficiency. It gives your plants direct access to essential nutrition. This product is beneficial for a nutrient lockout or damaged roots where leaves struggle to access minerals.


Add magnesium and calcium if your tank’s PPM levels are below 150. Check the water pH is above 5.8, flush it out, and replace it with pH-balanced H2O and marijuana nutrients.

Use a foliar spray with 0.06–0.13 fl. oz. of CMX per one gallon of water with a pH of 6.0. This product prevents the herb from translocating excess Mg by giving top leaves direct access to nutrients. 

It protects bottom leaves from necrosis and keeps foliage healthy while the plant heals. Only use this spray when the lights are off, as it could burn your crops, especially with inadequate humidity for cannabis growing.

Coco coir

Flush thoroughly and replenish with nutrient solution to hit 6.0 pH. It’s difficult for greenery to absorb the minerals in this medium. Consider a foliar spray and regular feeding to ensure your flora is healthy.

Use buffered coco coir only, as the unbuffered version may dry the roots and make nutrient absorption extra tricky. 

Steps to prevent a magnesium deficiency

Before letting it get to this stage, try to prevent the problem. Follow these tips to avoid a magnesium shortage in your marijuana crops:

Attain the ideal pH balance

Insufficient pH levels lead to low root temperatures, which causes acidic soil that struggles to absorb essential vitamins. Even if you have the best nutrients, your plant won’t reap the benefits with low pH.

Invest in a pH meter and regularly check the levels. Flush the medium and start again if it’s off balance, as it could lead to constant moisture in the soil. 

Use high-quality supplements

Poor-quality nutrients aren’t well balanced. They may have too much of one vitamin and too little of another, leading to a mineral excess during flowering. Natural nutrition, like eggshells, contains calcium but little magnesium. In these cases, it’s better to add an Mg supplement.

Don’t overwater

Overhydrating is a common error, especially with beginner growers. When watering outdoor weed, do it less often and consider the natural elements, like rain. 

Rotating between wet and dry soil is recommended because it helps their roots stretch deeper. When the earth is parched, it pulls in more oxygen. When it’s too wet, the roots drown and don’t get enough air.

Water your plants according to their life stages. Here’s how often to do it:

  • Germination: 4–7 days 
  • Seedling: 3–7 days 
  • Vegetative: 2–4 days
  • Flowering: 2–3 days

Use a topping spray

At the first signs of a magnesium deficiency in cannabis plants, use a foliar spray and flush the soil. This product helps the new growth remain unaffected by the problem. It’s especially beneficial if you’re close to the flowering stage, as you don’t want your harvest to suffer.

Make an Mg foliar treatment by combining one tablespoon of Epsom salt with about one gallon of water. Put it in a bottle and get another one with plain water. Spray your plants with the Mg solution every three days. Mist them with clean water every ten days to prevent residual buildup. 

Magnesium toxicity in cannabis

Cannabis plants can’t live without magnesium but also suffer from too much of it. What happens when your greenery experiences an excess? It increases nitrogen uptake, which is necessary, but it also affects calcium absorption in high amounts. 

Left untreated, it leads to Ca and Mg lockout, slowly starving your plants. Here are some signs that reveal magnesium toxicity in cannabis:

  • Dark brown spots on fresh growth 
  • Older leaves wither and fall off in unusual amounts 
  • New leaves are smaller and firmer 
  • Leaf tips curl or turn brown 
  • Leaves turn a black-brown color (a sign of severe magnesium toxicity, so act fast) 

A magnesium deficiency is an easy fix

A magnesium deficiency can have severe effects on your plant’s growth. On the bright side, it’s easy to fix if you catch it early. If you notice signs like yellow leaves and red stems, flush and renourish your crops immediately. 

Remember not to go overboard with Mg supplements, as too much is toxic to your flora. Grow fat, juicy buds by following our preventative steps and keeping an eye on your crops. 

The best tip is to start with high-quality seeds to ensure you get an all-around batch of healthy plants. Shop our selection of top-shelf cannabis seeds and reap the resinous rewards.

AUTHORED BY: Douglas Kester Mr. Kester came to i49 with a wealth of experience. He’s worked in the cannabis industry for more than ten years. Unsurprisingly, he’s a good friend of the Bennett siblings. Their love for this green herb and all its intricacies has fostered a close friendship. They share the same vision at i49, bringing you all the information you need to reap the full benefits of marijuana. As a growing expert at i49, Mr. Kester finds it hard to choose a favorite strain. Instead, he regards each one as unique and full of potential. Douglas finds it rewarding to experiment with specific cultivars and cross-breed to discover a new one.

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