Firstly, using High Times IMO is a biased source. They are driven by advertising dollars and by publishing an article that stated "MG is just fine for your cannabis" is clearly not in the best interest of a magazine depentent on cannabis specific sponsorships...
Secondly, independant research published in this article of the seattle times states the opposite of what you are saying;
http://community.seattletimes.nwsource.com/archive/?date=20000326&slug=4011973
For those who don't want to read the article...
"Alaska Fish Fertilizer products, widely sold in organic food stores, were a little over background level in arsenic and mercury, as you might expect from fish, but still well within the state standards.
A company called Down to Earth Distributors Inc. had mixed results with cadmium, mercury and lead. Blood Meal, Earth Feather Meal, Cottonseed Meal, Bio-Turf and Rhododendron Azalea Mix were cleaner than Vegan Mix, Aged Bat Guano, Seabird Guano and All-Purpose Vegetable 4-6-2, which were slightly above soil background level in two toxic metals.
Products from The Scotts Co., the largest fertilizer distributor to America's hardware and garden stores, were
very clean overall. Scotts submitted 163 test reports to the state; 84 were below the soil background level in all five toxic chemicals.
Scotts' Miracle-Gro was exceptionally low in contaminants. Nine of the 10 Miracle Gro products tested would have passed the strictest test proposed by environmentalists - a test opposed by Scotts, whose lobbyist argued there was no proof of harm from higher levels of toxic metals."
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Looks like there is a 0.5% Mg content to me...
Additionally, plants typically take up what they require out of the soil. Unless there is an excess of other nutrients or pH issue that block certain things from being draw out.
I was wrong Ca is not listed however, as stated on the package "is to be used as part of a complete system". Personally I use bone meal when transplanting anything, that adds plenty of Ca to the mix IMO.
I've had a veggie garden for years and never had a problem.
Again, this is my first mj grow so what do I know...