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Diagnosing and fixing a magnesium deficiency

Had a nasty mag deficiency show up mid-flower and wanted to document how I fixed it in case it helps anyone. Symptoms: yellow patches between the veins on older fan leaves, starting mid-canopy and working upward. Classic interveinal chlorosis. Cause: I was using RO water and hadn't been adding a cal-mag supplement. The low EC of RO water means plants can struggle to uptake secondary nutrients. Fix: Added 1ml/L of Calmag Plus to every watering for 2 weeks. New growth came in healthy green within 5-6 days. Older damaged leaves didn't recover but stopped getting worse. Lesson: if you're using RO or very soft tap water, always add cal-mag. Don't wait for deficiency signs.
9 replies
9 Replies
DP
Dan P.
17 Mar 2026
Really useful write-up, thanks for sharing. I've had similar results with the same approach.
DP
Dan P.
17 Mar 2026
Great looking grow! What nutrients are you running?
DP
Dan P.
17 Mar 2026
This is exactly the info I needed β€” saved this post for my next grow.
DP
Dan P.
17 Mar 2026
Congrats on the harvest! First grows are always special regardless of the yield.
DP
Dan P.
17 Mar 2026
I had the same deficiency issue last run. Cal-mag sorted it out within a week for me too.
DP
Dan P.
17 Mar 2026
What's your VPD sitting at during flower? Looks like a nice environment.
DP
Dan P.
17 Mar 2026
Trichomes are looking incredible. How much longer do you think she needs?
DP
Dan P.
18 Mar 2026
I've been on the fence about this strain β€” this review has convinced me to give it a go.
DP
Dan P.
18 Mar 2026
Love seeing grow diaries like this. Keep the updates coming!

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