Cannabis Magnesium Deficiency
Cannabis magnesium deficiency occurs when a plant cannot absorb enough magnesium to support chlorophyll production and photosynthesis. Magnesium is a secondary macronutrient and a core component of chlorophyll, making it essential for healthy green growth. Deficiencies are especially common in coco, hydro, and high-intensity LED grows.
What Is Magnesium Deficiency in Cannabis?
Magnesium deficiency in cannabis happens when magnesium is either missing from the nutrient solution or blocked due to pH imbalance or nutrient antagonism. Unlike calcium, magnesium is mobile, meaning the plant can move it from older leaves to new growth.
Because of this, symptoms usually appear first on lower and middle leaves, not new growth.
Cannabis Magnesium Deficiency Symptoms
Common signs of magnesium deficiency in weed include:
- Interveinal yellowing (yellow between veins, green veins remain)
- Yellowing starts on lower or mid-level leaves
- Rust or brown spots may appear as deficiency progresses
- Leaves may curl upward at the edges
- Reduced photosynthesis and slowed growth
- Leaves may eventually die and drop
Key identifier: yellowing between veins on older leaves, not spotting on new growth.
Magnesium Deficiency vs Calcium Deficiency
These two are often confused and grouped as “cal-mag,” but the symptoms differ clearly:
| Magnesium Deficiency | Calcium Deficiency |
|---|---|
| Affects older leaves | Affects new growth |
| Interveinal yellowing | Rust spots and deformation |
| Mobile nutrient | Immobile nutrient |
| Photosynthesis issue | Structural issue |
Knowing where symptoms appear is the fastest way to tell them apart.
What Causes Magnesium Deficiency in Cannabis?
Magnesium deficiency is commonly caused by:
- Incorrect pH blocking magnesium uptake
- Coco coir binding magnesium
- Reverse osmosis (RO) or soft water
- Excess potassium or calcium competing for uptake
- High EC or salt buildup
- Intense LED lighting increasing demand
Potassium-heavy bloom feeds are a frequent trigger during flowering.
How to Fix Magnesium Deficiency in Cannabis
To correct magnesium deficiency:
- Check and adjust pH
- Soil: ~6.2–6.8
- Coco/Hydro: ~5.8–6.2
- Add a magnesium source
- Cal-Mag supplement
- Epsom salt (magnesium sulfate) for quick correction
- Reduce excess potassium if present
- Flush and re-feed if lockout is suspected
- Resume balanced feeding at moderate strength
⚠️ Damaged leaves won’t recover — improvement shows in new healthy growth.
Can Magnesium Deficiency Occur During Flower?
Yes — and it’s common.
- Flowering increases magnesium demand
- Potassium competition intensifies
- Deficiency during flower reduces photosynthesis
- Severe cases can limit bud size and resin production
Many growers continue light magnesium supplementation through early and mid-flower.
How to Prevent Magnesium Deficiency
- Buffer coco properly before use
- Supplement magnesium when using RO water
- Avoid excessive potassium feeding
- Monitor runoff EC and pH
- Increase Cal-Mag slightly under LEDs
- Feed consistently rather than aggressively
Quick Summary
- Magnesium is essential for chlorophyll and photosynthesis
- Deficiency affects older leaves first
- Interveinal yellowing is the clearest sign
- Coco, RO water, and LEDs increase risk
- Often paired with calcium issues but visually distinct