A magnesium deficiency sometimes happens after supplementing plants with something that contains calcium but not magnesium, such as agricultural lime (non-dolomite lime), egg shells, etc. This won’t happen with proper cannabis nutrients. If you’re using good nutrients, cannabis plant may still show signs of a magnesium deficiency if the pH at the roots is too low, especially in hydro. That is because when the pH of your root zone is not in the correct range, your cannabis cannot properly absorb magnesium through the roots.

Flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients that includes magnesium. This will remove any nutrient salts that may be affected the uptake of magnesium and help restore pH to the proper levels.

NOTE: Once a magneisum deficiency is cleared up, the problem (yellowing lower leaves) will stop spreading to other older leaves, usually within a few days. Leaves which have been damaged by a magnesium deficiency will probably not recover or turn green, so you want to pay attention to other growth for signs of recovery.