de
en
Reading time: approx. 6 to 8 minutes
Magnesium deficiency in cannabis usually appears first on lower, older leaves because magnesium is mobile within the plant. The plant reallocates magnesium from older tissue to support new growth.
This guide explains symptoms, causes, treatment, and prevention and complements the overview in All cannabis nutrient deficiencies. For general yellowing patterns, also see yellow leaves on cannabis plants.
The role of magnesium
Magnesium is a core component of chlorophyll and therefore directly involved in photosynthesis. It also activates numerous enzymes and stabilizes key metabolic processes.
When magnesium is not sufficiently available, plants appear pale, nutrient efficiency declines, and overall physiological balance is disturbed.
Rule of thumb: If older leaves yellow between the veins, magnesium is a realistic candidate.
Signs and typical symptoms
- Interveinal chlorosis on older leaves.
- Yellow leaf tissue with green veins.
- Rust colored spots or small necrotic patches over time.
- Brittle or slightly curling leaves.
- Slowed growth in advanced stages.
New growth often remains green at first, clearly distinguishing magnesium deficiency from immobile nutrient issues.
Common causes
- pH outside the optimal uptake range.
- High EC or salt buildup causing nutrient lockout.
- Nutrient antagonisms, especially excess calcium or potassium.
- Very soft water lacking base mineral content.
- Waterlogging and poor root zone aeration.
In many cases, magnesium is present but unavailable to the plant.
Step by step treatment
- Confirm the diagnosis: Symptoms affect older leaves first.
- Check pH and EC: Stabilize uptake conditions before adding nutrients.
- Review antagonisms: Consider calcium and potassium ratios.
- Supplement carefully: Add magnesium moderately and monitor response.
- Stabilize irrigation: Avoid water stress and saturation.
Once uptake is restored, new growth usually normalizes. Damaged leaves do not recover.
Prevention and stable routines
- Maintain stable pH values within the optimal range.
- Avoid running EC permanently at the upper limit.
- Balance nutrient ratios consistently.
- Maintain substrate structure and proper drainage.
- Match water quality to medium and setup.
Classification and common mix ups
Magnesium deficiency is often confused with potassium deficiency. The key difference is the pattern: magnesium causes interveinal chlorosis, while potassium primarily damages leaf margins.
For clearer differentiation, see Nutrient deficiency vs overfertilization.
Important note:
If magnesium deficiency is suspected, restoring nutrient uptake should come first. Often the issue is not a true deficiency but a pH or EC imbalance. Suitable products can be found in the Magnesium nutrient category.
Conclusion:
Magnesium deficiency typically starts on older leaves and appears as interveinal chlorosis. In most cases, the root cause is impaired uptake due to pH drift, EC issues, or nutrient imbalance. Diagnose first, then correct deliberately to avoid secondary damage.