There’s seems to be a lot of speculation on the web whether Epsom salt is really useful for container gardening or whether it boils down to some sort of old wive’s tale. There really needs to be no mystery about its usefulness, the primary ingredient is magnesium sulfate, and plants definitely need magnesium.
Now if you are growing in the ground a magnesium deficiency may be less of an issue, but since this blog is geared specifically towards container gardening magnesium is pretty much essential. If you are gardening in containers you need to provide all of the nutrients and especially micro-nutrients the plant needs to thrive. Not many of the fertilizers I have seen contain magnesium which is why Epsom salt can come in so handy.
You just add a tablespoon per gallon of water, and may use it in foliar feeding as well. Adding magnesium can prevent nutrient lockout as well, so the nutrients already existing become available.
This is part of the reason it is good to feed your plants their scheduled fertilizer right after you provide them with the Epsom salts.