Magnesium deficiency in cannabis
What it looks like
Yellowing between the veins on older, lower leaves, while the veins stay green. Edges can bronze later. Common when the root-zone pH is off and the plant cannot take up what is already in the medium.
What usually causes it
Confirm pH before you add anything. If pH is out of range, fix that first. Epsom salt is the usual magnesium source growers use, at the rate on the bag, not a guess. Do not stack it on top of a full bottle feed.
Check this first
Look at a lower leaf. Yellow between the veins, veins still green. Before Epsom salt, read the runoff pH. Soil wants about 6.0 to 6.5. If the pH is off, the magnesium is probably already in the pot and the root cannot take it. Fix the pH, then wait a few waterings. Add magnesium only if the pH was already in range.
This is identification for a home grow. It is not a pesticide label and not medical advice. Follow your local law.