Magnesium deficiency in cannabis

Adam Parsons

Cannabis Journalist & Staff Writer at Zamnesia

Adam Parsons

Last updated: September 8, 2025

Are your cannabis leaves turning yellow between the veins? It could be magnesium deficiency. Here's how to fix it.

Magnesium deficiency in cannabis is a common yet often misunderstood issue, especially among home growers. An imbalance of this essential nutrient can compromise your plant's health, reduce yields, and impact bud quality when left unchecked. In this guide, we'll walk you through everything you need to know, from spotting early symptoms and confirming a diagnosis to applying fast, effective treatments and preventing future problems. Whether growing in soil or a hydro setup, we'll help you restore your plants to full vitality.

What is magnesium and why is it important?

Magnesium is one of the secondary macronutrients your cannabis plants need to survive and thrive. While it's required in smaller amounts compared to nitrogen, phosphorus, and potassium, magnesium plays a vital role in photosynthesis, the process by which plants convert light into energy. It forms the core atom of chlorophyll molecules, which gives leaves their green color and allows them to absorb sunlight.

Without enough magnesium, chlorophyll production slows and cannabis leaves begin to yellow (a condition known as interveinal chlorosis), starting with the oldest leaves near the bottom of the plant. This early sign is one of the first clues of magnesium deficiency in cannabis.

Magnesium plays a variety of other roles, such as:

• Activates key enzymes involved in energy production
• Helps with nutrient transport across plant tissues
• Supports healthy root development and nutrient uptake
• Affects bud formation, flavor, and overall yield

Although deficiencies are much more common in soil grow-ops, hydroponic setups can also be impacted if pH levels or nutrient concentrations fall out of balance. Understanding how and when your cannabis plants use magnesium is crucial for diagnosis and prevention early on.

Magnesium needs during pre-flowering and flowering stage

As cannabis plants move from the vegetative stage into pre-flowering and eventually full bloom, their demand for magnesium increases significantly. During flowering, plants grow larger and redirect energy towards bud development, resin production, and terpene expression, all of which rely on strong metabolic and photosynthetic function.

If magnesium levels are low during this period, the plant may:

• Show rapid chlorosis in older fan leaves
• Struggle with nutrient translocation to developing flowers
• Exhibit weakened stems and poor structural integrity
• Suffer from reduced bud density, size, and aroma

Ensuring adequate magnesium during flowering helps support lush, vigorous growth and maximizes the quality of your final harvest. This is why many growers proactively supplement with magnesium fertilizers when transitioning into the bloom phases, especially if they've had issues in past grows.

Early symptoms of magnesium deficiency

Magnesium deficiency in cannabis plants usually starts subtly, but if left untreated, it can escalate quickly. Early detection is key to avoiding long-term stress, poor yields, and unsightly buds.

Here are some of the most common early warning signs to look out for:

1. Yellowing between leaf veins (interveinal chlorosis)

The most telling symptom of magnesium deficiency is interveinal chlorosis, where the tissue between the veins of older fan leaves begins to turn yellow while the veins remain green. This happens because magnesium is mobile within the plant; when there's a shortage, it's pulled from older leaves and redirected to new growth.

2. Symptoms begin on lower/older leaves

Unlike many other nutrient issues, magnesium deficiency appears first on the oldest leaves, typically at the base of the plant. This can make it easier to distinguish from deficiencies in iron or calcium, which affect new growth first.

3. Brown spots and leaf curling

As the deficiency progresses, yellowed areas may develop rust-colored spots or edges. The leaf tips may begin to curl upward or downward, and affected leaves often feel brittle and dry.

Magnesium deficiency in cannabis typically progresses over several weeks. During week 1, you may notice some faint yellowing between the veins on the lower fan leaves. By week 2, this begins to spread, and the leaf tips might start to curl slightly.

Come week 3, brown spots may emerge on the yellowed areas, discoloration deepens, and more leaves may become affected. If left untreated by week 4 and beyond, the damaged leaves often begin to die off, with noticeable curling (often upward) or a dry, crumbling texture.

Symptoms may also start climbing to mid-level foliage. Many growers initially mistake these signs for nitrogen deficiency, so it's important to consider where the symptoms appear and how they evolve over time when diagnosing your plant.

Progress of a magnesium deficiency

How to confirm it's a magnesium deficiency

Before treating your plant, it's key to be certain you're dealing with magnesium deficiency in cannabis, and not a different issue with similar symptoms. Misdiagnosis can lead to unnecessary corrections that do more harm than good.

Rule out pH imbalance and nutrient lockout

One of the most common causes of magnesium deficiency symptoms isn't a lack of magnesium itself; it's a pH imbalance. If the pH of your growing medium is outside the optimal range, your plants may not be able to absorb magnesium even when it's present in the soil or solution. This condition is called nutrient lockout. Always test and correct your pH before adding any magnesium supplements. If lockout is the issue, simply flushing and rebalancing your medium might be all that's needed.

Visual confirmation vs other deficiencies

Magnesium deficiency can mimic other nutrient problems, especially nitrogen, calcium, or iron deficiencies. However, there are key differences:

• Magnesium: Affects older leaves first and causes yellowing between veins
• Nitrogen: Causes overall yellowing that begins at the bottom of the plant
• Iron: Starts on newer growth, with yellowing near the top of the plant
• Calcium: Often shows as spotty necrosis and weak stems, not interveinal yellowing

For a complete comparison of similar symptoms, check out our nutrient deficiencies guide.

Fixing magnesium deficiency in cannabis

Once you've confirmed magnesium deficiency in your cannabis plants, swift action is essential. The good news? It's relatively easy to correct, especially when caught early. Below are three key strategies to restore magnesium levels and help your plants bounce back to full health.

Adjust soil or hydro pH

The first step in treating magnesium deficiency is to ensure your pH levels are in the optimal range for nutrient uptake. If your medium is too acidic or alkaline, your plant may not absorb magnesium even if it's available.

• Soil: 6.0–6.5 pH
• Hydroponics: 5.5–6.0 pH

Use a reliable pH meter or test kit to check your medium. If the pH is out of range, flush with pH-balanced water and adjust with suitable products until it reaches the ideal zone. Fixing the pH is often enough to resolve mild magnesium deficiencies.

pH Test Strips (Zamnesia)

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pH Test Strips (Zamnesia)

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Use a magnesium supplement

If symptoms persist after correcting the pH, the next step is to add a magnesium-rich supplement. There are many different supplements available on the market to suit all preferences and budgets. One of the most effective and budget-friendly options is Epsom salts (magnesium sulfate), which can be used in both soil and hydro systems.

Application guidelines:

• Soil: Mix one teaspoon of Epsom salts per gallon of water and apply during your regular watering schedule.
• Hydroponics: Add 0.5–1 gram per 34 fl oz of reservoir water, depending on the severity of the deficiency.

Apply once and observe for several days. Repeat only if symptoms persist. Whether using supplements or Epsom salts, avoid overapplication as this can disrupt the balance of other nutrients like potassium and calcium.

Foliar spray for fast absorption

If your plants need a rapid magnesium boost, especially during flowering, foliar feeding effectively delivers nutrients directly to the leaves.

How to apply a magnesium foliar spray:

1. Dissolve one teaspoon of Epsom salts in 34 fl oz of warm water.
2. Stir well, allow the solution to cool, then pour into a clean spray bottle.
3. Lightly mist the tops and undersides of affected leaves.
4. Spray during low-light periods (early morning or evening) to prevent light burn.

Foliar sprays can provide visible improvements within 24–48 hours, but should be combined with root feeding to fully resolve the deficiency.

Ultra Fine Misting Bottle (Zamnesia)

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Ultra Fine Misting Bottle (Zamnesia)

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Can I still use affected plants or buds?

One of the biggest questions growers face when dealing with magnesium deficiency in cannabis is whether the end product will still be safe and enjoyable. Fortunately, the answer is usually yes, as long as the issue is addressed before harvest and doesn't lead to widespread plant stress or disease.

Is it safe to consume?

Yes, consuming buds from plants that experienced magnesium deficiency is typically safe. Magnesium by itself is not harmful to humans, and a deficiency in the plant doesn't introduce toxins or make the buds dangerous. However, if the plant has suffered severe stress or developed mold or mildew due to weakened health, that could present issues, so visual inspection at harvest is always essential.

Impact on potency, yield, and flavor

While safety usually isn't a concern, quality may be affected in several ways:

• Potency: If the plant was stressed during critical phases (especially flowering), cannabinoid production may be reduced, leading to less potent buds.
• Yield: Magnesium deficiency disrupts photosynthesis, slowing growth and bud development. As a result, you may see smaller or lighter flowers.
• Flavor and aroma: A stressed plant produces fewer terpenes, resulting in bland or muted flavors compared to healthy buds.

The earlier you identify and fix the issue, the more you can preserve the quality of your harvest. Mild to moderate magnesium deficiencies often have minimal long-term impact, especially if corrected during early to mid-flower.

Preventing future magnesium deficiency

Preventing magnesium deficiency in cannabis is all about maintaining stable and healthy growing conditions. Once you've corrected a deficiency, staying ahead of the problem will help ensure vigorous growth and maximize performance and overall yields.

Here's how to prevent magnesium deficiency, using some simple, effective habits:

• Keep pH in check: Magnesium becomes unavailable when pH falls outside the ideal range. Test regularly and be sure to keep levels around 6.0–6.5 for soil and 5.5–6.0 for hydro.
• Don't overfeed: Avoid excess calcium, potassium, or ammonium, which can interfere with magnesium uptake.
• Test your soil: Use home kits or occasional lab testing to monitor nutrient balance, especially during active growth phases.
• Start with quality media: Use well-buffered soil or coco coir enriched with trace minerals. If soil is being reused, flush and replenish it between grows.

With some foresight and regular monitoring, you can prevent most nutrient imbalances, including magnesium deficiency, and enjoy healthier, more productive plants.

Magnesium: Crucial to plant growth

Magnesium may be a secondary nutrient, but its role in cannabis cultivation cannot be understated. From fueling photosynthesis to supporting healthy bud formation, it's a key player in keeping your plants vigorous and resilient and helps them to flourish. By recognizing the signs of magnesium deficiency early and taking a balanced, preventative approach, you'll set your grow up for success, not just now, but for future harvests too.

Looking to grow stronger, more nutrient-efficient plants from the start? Explore our range of high-quality cannabis seeds and find genetics built for performance and resilience.