Calcium and Magnesium Deficiency in Cannabis
Yellow between the veins, rusty spots, weak new growth — fix cal-mag issues and pH before your buds suffer.
Why cal-mag issues are so common
Calcium builds cell walls; magnesium sits at the center of chlorophyll. LED-heavy grows, coco, and RO water all increase demand — your base bloom formula may not keep up.
Many "mystery" deficiencies in mid-flower are cal-mag or pH lockout masquerading as something else. Check both.
Calcium deficiency signs
Calcium moves slowly in the plant — symptoms hit new growth first.
- Rusty brown spots on young leaves and leaf margins
- Cracked or distorted new leaves
- Weak stems and tip burn on fresh growth
- Root tips brown in severe cases (hard to see without uprooting)
Magnesium deficiency signs
Classic interveinal chlorosis — yellow between veins while veins stay green, often on older leaves first (mobile element).
- Yellow V-shaped pattern marching up from lower leaves
- Older fan leaves fade while new growth still looks OK initially
- Purple or red stems in some strains under stress
- Often appears in week 4–6 of flower when demand peaks
Pro tip
A foliar spray of diluted cal-mag can confirm Mg issue within days — if new growth greens up, you found it.
Fix and prevent
Add cal-mag supplement to your feed at label dose for coco and RO; half to full dose in soil depending on water hardness. Hard tap water often supplies enough calcium — test before doubling cal-mag blindly.
Fix pH first if runoff is out of range — Mg locks out above 6.5 in soil and below 5.5 in coco.
Cal-Mag Plus
Calcium, magnesium, and iron supplement. Essential add-on for coco, RO water, and LED-heavy grows.
Best for
Coco, RO water, or any grow showing calcium/magnesium deficiency
Why we recommend it
Coco binds calcium aggressively; RO strips minerals from tap. Cal-mag prevents the classic new-growth yellowing that base nutrients alone won't fix.
CALiMAGic
Calcium and magnesium supplement from the same line as Flora. Low N, easy to add alongside any base nutrient program.
Best for
Flora users and anyone needing a reliable cal-mag backup
Why we recommend it
Pairs cleanly with Flora Trio and most liquid programs. Keep a bottle on hand even in soil if you use soft water or heavy coco blends.
Notes by medium
Cal-mag demand varies:
- Coco: cal-mag every feed from seedling — non-negotiable
- RO water in any medium: supplement Ca/Mg every feed
- Soil with lime/charged mix: often lighter cal-mag needs until mid-flower
- LED vs HPS: LED grows report higher cal-mag demand anecdotally — watch new growth
Disclaimer: This article is for educational purposes only. Cannabis laws vary by jurisdiction. Always comply with local regulations.