A calcium deficiency is common because many hydroponic fertilizer products contain very little calcium, plus in some regions there isn’t very much of it native in the tap water either. If calcium is deficient in both the tap water and the fertilizer a calcium deficiency is probable.
The first signs are yellowing on the new shoots, followed by brown spots appearing on younger leaves.
Older leaves start to show symptoms after damage has already appeared on the new growth. They turn dark green while stems weaken. Browning starts along the leaf edges before advancing inward and killing the leaf.
Cal-Mag can be used as a foliar spray and in solution to correct a calcium deficiency. It can be used in solution during both flowering and vegetative growth. Only use Cal-Mag as a foliar spray during vegetative growth.
A calcium toxicity blocks out potassium and magnesium, causing symptoms that look similar to a deficiency of either of these two minerals.