A magnesium deficiency in a marijuana plant can be a serious problem. When a plant is suffering from it, the veins of the leaves will start to look light green or yellow in color, while the areas between the veins may turn pale or develop rust-colored spots. This creates a clear contrast from the healthy, vibrant green leaves cannabis plants should have.
If not treated early, the plant may lose a significant number of leaves, which can reduce growth and negatively impact yields. Fortunately, magnesium deficiency in cannabis is relatively easy to fix once correctly identified. This article covers all you need to know about magnesium and cannabis plants, and how to identify and solve a magnesium deficiency.
Key Takeaways
- Calcium and magnesium are secondary nutrients
- Magnesium helps regulate the uptake of other nutrients, including phosphorus
- Magnesium is a core component of chlorophyll, which drives photosynthesis
- Magnesium supports enzyme function and nutrient transport
- Magnesium improves resilience to environmental stress, such as drought
- Deficient plants produce yellowing between the veins of older leaves
- Deficient plants produce rusty or brown spots developing on affected leaves
- Deficient plants produce leaves that become dry, brittle, or curling upward
- Older leaves become brittle and begin dropping off deficient plants
- Key Takeaways
- Quick Diagnosis: Is This Magnesium Deficiency?
- Why Do Marijuana Plants Need Magnesium?
- Magnesium plays several key roles in plant health:
- Magnesium Deficiency in Coco Coir
- What Are the Signs of Magnesium Deficiency?
- What Causes Magnesium Deficiency?
- Magnesium vs Calcium vs Nitrogen Deficiency
- How to Fix Magnesium Deficiency in Cannabis
- Growing in Hydroponics
- Magnesium When Growing Autoflowers
- Preventing Magnesium Deficiency
- Cal-Mag: Should You Use It?
- Final Thoughts
- Frequently Asked Questions About Magnesium Deficiency in Cannabis Plants
Quick Diagnosis: Is This Magnesium Deficiency?
If you do not know if your plants are not getting enough magnesium, then use this checklist to quickly confirm whether your cannabis plant is suffering from a magnesium deficiency. If most of these signs match your plant, it is quite likely you are dealing with a magnesium deficiency.
- Your pH may be below 5.5 (coco/hydro) or 6.0 (soil)
- Symptoms start on older, lower leaves first
- Yellowing appears between leaf veins (not uniform yellowing)
- Leaf veins remain green while the surrounding areas turn yellow
- Rusty or brown spots begin to develop over time
- Leaves may become dry, brittle, or curl upwards
- The issue appears during mid-veg or early flowering
- You are growing in coco coir or using soft water
Why Do Marijuana Plants Need Magnesium?
Magnesium is a mobile nutrient in cannabis plants. This means the plant can move it from older leaves to newer growth when supplies are limited. Because of this, deficiencies almost always appear on older, lower leaves first. Magnesium is a secondary nutrient with its counterpart calcium.
Magnesium plays several key roles in plant health:
- Helps regulate the uptake of other nutrients, including phosphorus
- A core component of chlorophyll, which drives photosynthesis
- Supports enzyme function and nutrient transport
- Improves resilience to environmental stress, such as drought
Cannabis plants require magnesium throughout their lifecycle, but demand often increases during the flowering stage, when plants are producing larger buds and require more energy for developmentagnesium is a mobile nutrient in cannabis plants. This means the plant can move it from older leaves to newer growth when supplies are limited. Because of this, deficiencies almost always appear on older, lower leaves first.
Magnesium plays several key roles in plant health:
- It helps regulate the uptake of other nutrients, including phosphorus
- It is a core component of chlorophyll, which drives photosynthesis
- It supports enzyme function and nutrient transport
- It improves resilience to environmental stress, such as drought
Cannabis plants require magnesium throughout their lifecycle, but demand often increases during the flowering stage, when plants are producing larger buds and require more energy for development
Magnesium Deficiency in Coco Coir
Magnesium deficiency is more common when growing in coco coir. This is because coco coir does not naturally retain calcium and magnesium well, and can bind to these nutrients, making them unavailable to the plant. You will actually need to buffer some coco coir before use to ensure the adequate levels of calcium and magnesium are present.
For this reason, growers using coco coir should:
- Ensure nutrients are properly balanced
- Use a Cal-Mag supplement regularly
- Monitor pH closely
- Make sure they coco coir has been well buffered
Actionable Tip: Include a Cal-Mag supplement in every feed, as coco does not retain these nutrients well
What Are the Signs of Magnesium Deficiency?
Magnesium deficiency does not appear overnight. It typically develops gradually, often becoming visible during mid to late veg or early flowering. Below is a list of common symptoms to be on the look out for if you think your plants may be suffering from a magnesium deficiency.
Common Symptoms
- Yellowing between the veins of older leaves (interveinal chlorosis)
- Rusty or brown spots developing on affected leaves
- Leaves becoming dry, brittle, or curling upward
- Older leaves become brittle and begin dropping off
- Overall, the plant looks weak and stressed
Once visible, the deficiency has already been affecting the plant for some time, so early action is important.
What Causes Magnesium Deficiency?
There are several common causes of magnesium deficiency when cultivating cannabis. Below explains the most common causes and how to avoid them.
pH Imbalance (Most Common Cause)
If the pH is too low or too high, the plant cannot absorb magnesium , even if it is present. This is known as nutrient lockout and cuts the plants off from the available nutrients
- Soil: optimal pH 6.0–7.0
- Hydro/coco: optimal pH 5.5–6.5
Actionable Tip: Always check pH before increasing nutrients, as imbalance blocks magnesium uptake
Lack of Magnesium in Nutrients
Some feeding schedules may not provide enough magnesium, especially in fast-growing plants or heavy feeders.
Overwatering
Excess watering can flush nutrients from the root zone and reduce oxygen availability, limiting nutrient uptake.
Coco Coir Without Supplementation
As mentioned, coco requires additional calcium and magnesium to avoid deficiencies, and should always be buffered before using.
Magnesium vs Calcium vs Nitrogen Deficiency
Magnesium deficiency is often confused with other nutrient issues. Correct diagnosis is key.
| Deficiency | Where It Appears | Key Symptoms |
| Magnesium | Older leaves | Yellowing between veins, rust spots |
| Calcium | New growth | Twisted leaves, brown spots, stunted growth |
| Nitrogen | Older leaves | Uniform yellowing (not between veins) |
Magnesium affects older leaves first, while calcium affects new growth, this is one of the easiest ways to tell them apart.
How to Fix Magnesium Deficiency in Cannabis
Luckily, magnesium deficiency is straightforward to treat when addressed early. Below is a step-by-step guide on how to fix a magnesium deficiency.
Step 1: Check and Correct pH
Before adding nutrients, always correct the pH using a digital pH pen for the most accurate reading.
- Soil: adjust to 6.0–7.0
- Hydro/coco: adjust to 5.5–6.5
If pH is off, adding more nutrients will not solve the issue and you will need to manually change the pH level using pH up or down products.
Step 2: Add Magnesium
The most common and effective solution is Epsom salts (magnesium sulfate). You can also use a liquid version of magnesium and follow the recommended feeding guide line.
Recommended Dosage:
- Soil / watering: 1 teaspoon per litre of water
- Foliar spray: 0.5 teaspoon per litre (spray leaves lightly)
- Hydro: add to reservoir and monitor EC
Epsom salts dissolve easily and provide a fast-acting magnesium source.
Actionable Tip: Avoid overuse, as excess magnesium can lead to nutrient imbalance
Step 3: Flush (If Needed)
If nutrient buildup or lockout is suspected:
- Flush the growing medium with pH-balanced water
- Then reintroduce nutrients at the correct levels
Check runoff pH to ensure the medium is stable. This is done by collecting the run off nutrient at the base of the pot or inside the hydro system, and using a pH pen to check.
Step 4: Monitor Recovery
After treatment:
- New growth should appear healthy within 3–7 days
- Damaged leaves may not fully recover but should stop worsening
Once the issue is resolved, you can remove heavily damaged leaves if necessary, as these are prone to attracting insects.
Growing in Hydroponics
For hydroponic growers:
- Check reservoir pH and EC levels
- Replace the nutrient solution if imbalanced
- Add magnesium (Epsom salts or Cal-Mag) as needed
Maintaining a stable reservoir is key to preventing recurring issues.
Magnesium When Growing Autoflowers
Magnesium plays a central role in photosynthesis by forming part of chlorophyll molecules. Without enough magnesium, autoflowers cannot efficiently convert light into energy.
However, autoflowers require lower nutrient levels overall due to:
- Smaller size
- Faster lifecycle
- Reduced root mass
Overfeeding magnesium can lead to nutrient imbalances and reduced growth.
The key is to provide a balanced, reduced-strength feed and monitor plants closely rather than overcorrecting.
Preventing Magnesium Deficiency
Preventing magnesium deficiency is always easier than treating it so to avoid future issues make sure you properly organize your grows, give the right amount of water, test soil, and get your nutrients calculated before you plant your seeds.
Here are 5 key practices you can put into place to prevent magnesium deficiency:
- Use a pH meter and maintain correct pH levels at all times (ideal pH level varies depending on if you are growing in soil, hydro, coco so be sure to check before you start)
- Use a balanced nutrient schedule and follow the recommended ratios.
- Add Cal-Mag when growing in coco and buffer if necessary
- Avoid overwatering by testing if the growing medium is heavy or light
- Regularly inspect older leaves for early signs
Cal-Mag: Should You Use It?
Many growers treat magnesium deficiency using Cal-Mag supplements. While this can be effective, it is not always necessary.
Magnesium and calcium deficiencies are different:
- Magnesium → affects older leaves
- Calcium → affects new growth
Some growers see great results using Cal-Mag, while others find it can lead to excess nutrients if not needed.The best approach is to diagnose correctly before treating, and use a separate source of magnesium to treat a magnesium deficiency.
Final Thoughts
Magnesium deficiency is one of the more common issues cannabis growers face, but it is also one of the easiest to fix with the right approach.
By understanding how magnesium works in the plant, monitoring pH closely, and responding quickly to early symptoms, growers can prevent long-term damage and keep plants healthy throughout their lifecycle.
Frequently Asked Questions About Magnesium Deficiency in Cannabis Plants
1. How long does it take to fix magnesium deficiency in cannabis?
Most plants begin to show improvement within 3–7 days after treatment, although damaged leaves may not fully recover.
2. What is the fastest way to fix magnesium deficiency?
Correct the pH first, then add magnesium using Epsom salts or a liquid supplement for fast absorption.
3. Can cannabis plants fully recover from magnesium deficiency?
Yes, if treated early. Severe deficiencies may impact yield, but plants can still complete their lifecycle.
4. Should I remove leaves affected by magnesium deficiency?
Wait until the issue is resolved. Remove only heavily damaged leaves to prevent stress and pest attraction.
5. Why is magnesium deficiency common in coco coir?
Coco coir binds calcium and magnesium, making them unavailable unless supplemented properly.
6. Can overwatering cause magnesium deficiency?
Yes. Overwatering reduces oxygen in the root zone and can wash away nutrients, limiting uptake.
