Magnesium (Mg) Deficiency is quite common. (Mg) deficiency usually turns the older growth leaves a spotty or blotchy yellow. It will eventually get brownish as it worsens. Usually it starts at the outer edges of the leaf and works it way in. The margins of the leaves may bulge up and the tips can curl.
To correct the problem foliar feed the plants by misting. Add 1/4 – 1/2 tablespoon Epson salt diluted in warm water. Then add Epson salt…. around a 1/2 – 1 tablespoon per gallon of water to your reservoir.
Magnesium defficiency is a very common problem when growing marijuana. The cannabis plant uses a lot of (Mg) and many fertilizers do not provide enough.