Magnesium(Mn) is a micro nutrient but also an immobile element that uses Nitrogen(N) right out of the air. It can show up on your plants young leaves as a mottled yellow and brown areas. Dead yellow spots will form on the top leaves while grey spots can gather on the older ones. It is also like some of the other micro nutrients in that it doesn't respond well to a high PH. You can not only clear this up by watering with a fertilizer but plants respond well to being foliar fed their Manganese(Mn) also. Regular manure for your garden like cow manure contains needed doses of this nutrient for your Cannabis plants.
Getting some fertilizer and nutrients along with some helpful advice can be accomplished anywhere along Highway 101 from our great stores we have to help us grow our weed.
Nick At Garden all years says he likes Azomite for all his micro nutrient needs including Maganese(Mn). It's a fantastic and organic way to take care of many of our plants little micro problems. He also uses Dyna-Gro Foliage Pro foliar spray because of the way it gives a more immediate boost to get your plants right back on track.
In San Luis Obispo at the Healthy Harvest store they recommend getting one of their fine micro-nutrient products which have small amounts of all the micros your plant could need including Sulfur(S). In particular they like the CNS17 from Botanicare. It works out real nice that as long as your plants aren't flowering it works as an excellent foliar feed so the results will show up much quicker. You can also use this as an additive to your water which works good too.