Prepper Guide says you actually can over do it... You'd have to overdo it a lot to actually harm most plants, but why waste time and product?

Plants basically need 17 elements which are found on the periodic table. And the plants need to be fed the correct ratio in the proper way. I'm not here to give a lesson on agronomy, but unless a soil test indicates a low level of magnesium you don't need to apply more than an 8th of a cup per 5gal bucket per harvest.