Cal-Mag Deficiency: Rust Spots and Yellow Veins
By The Budometry TeamPublished Updated
Calcium and magnesium deficiencies are the most common problems in modern home grows, largely because RO water and high-intensity LEDs both increase demand.
Part of our pillar guide: Watering and Feeding Cannabis: Water Types, EC and Schedules
Calcium deficiency
Rusty brown spots scattered on newer, upper leaves, often with distorted or hooked new growth. Calcium is immobile, so damage shows on new growth first and never heals on affected leaves.
Magnesium deficiency
Yellowing between the veins on older, lower leaves while the veins themselves stay green. Magnesium is mobile, so the plant strips it from old leaves to feed new ones.
Dosing
Typical Cal-Mag supplements run 3-5 ml per gallon. RO and distilled users should add it to every feed. Hard tap water users often need none and can overdo it.
Check pH first
High pH in soil locks out magnesium and low pH locks out calcium. If your pH is off, adding more Cal-Mag makes salt buildup worse without fixing anything.
Frequently asked questions
- Can I use Epsom salt for magnesium?
- Yes, about a quarter teaspoon per gallon adds magnesium and sulfur, but it does nothing for calcium.
The full guide on this topic
Watering and Feeding Cannabis: Water Types, EC and Schedules
Which water to use — RO, city, well, softened or distilled — plus EC and ppm targets by stage, Cal-Mag rules, runoff percentage, and how to fix nutrient burn.