Magnesium sulfate (epsom salt) isn't really a "fertilizer" as it has no NPK. It is more of a mineral additive, magnesium and sulfur. It is used mainly on tomatoes, peppers and eggplant although it will not hurt to use it on just about everything. In alkaline soils it really helps plants in the uptake of calcium. I add 1/2 cup around the base of my plants at planting and about 1/4 cup per month during the growing season. It is essential in my soils as it eliminates blossom end rot and it also helps in increasing blooms. I wouldn't use it as a foliar more than twice a month
Magnesium sulphate also known as Epsom salts is an important fertilizer for production of the green chlorophyll in plants. Plants with magnesium deficiency have a yellowish coloration otherwise known as leaf chlorosis. This results in poor yields as plants can't make enough food. It should be applied on the soil or as as a foliar feed.