Hydrated magnesium sulfate in the form of white crystalline salt – epsom salts add magnesium to the soil.
Merriam-Webster Online Dictionary
Epsom salts (noun plural but singular or plural in construction)a bitter colorless or white crystalline salt MgSO·7HO that is a hydrated magnesium sulfate with cathartic properties - 4 2
Magnesium sulfate, MgSO4, bitter salts, bath salts, Epsomite
