How is Epsom salt used for watering?

It is plant available when it is dissolved into water... no microbes or additional efforts needed. using guaranteed analysis labels you can see the % of mass for Mg and S. You can use the cheapest brand foodgrade epsom salt, too. it does not need to be anything 'special' ... just no scents/aromas or other added nonsense. pure epsom is always pure epsom.. there are absolutely no quality differences in epsom salt. it has a chemical equation.. either it is magnesium sulfate or it is not. nothing between. how much you need depends on othe components... even your tap water may impact how much you need to give, along with all the other relevant envionmental variables and other things that impact growth rates. So, in total i give about 85ppm of Mg and 110ppm of S. I have very hard water. How much does my tap supplement? that may cause a variance in how much you need with Mg moe likely than S.. think S is only a few ppms in tap water? but mg can be higher. Tap has varying amounts of Ca too, but irrelevant when looking at epsom specifically, since it only adds mg and s. you can use an online calculator to tabulate all the ppms of each molecule you add. if in soilless/hydro, i'd reccommend it. If in soil, you need to use this information differently... soil is not as pecise. you may need to give more if only irrigating 1/2 or 1/3 times, for example. but, the rate of use over long-run will be similar, relative to growth rate in you garden.