Trying to get more magnesium into my dad, but he's not so hot on the supplement idea, and doesn't really buy the "depleted soil" theory. His question, if said depleted soil thing isn't a variable, is "What did people do for magnesium in the old days?"
Seemed like a fair enough question to me, so I will ask it on his behalf.
I have some theories about people visiting therapeutic mineral baths, spending time seaside where you'd be exposed to it from the sea spray or wading out into the water itself, drinking mineral water from special springs, unrefined sea salt, bone broths, adding lots of kale and other greens to soups, eating fish.
My thoughts for increasing his magnesium right off the bat would be to juice fresh veggies, to get a concentrated dose without having to eat his body weight in vegetables every day, and to make him fresh soups with bone broth at least once a week. I think he's already using Celtic Sea Salt.