Skip the ash in container soils - too high in soluble salts other than the potash it supplies. Whether or not you need Epsom salts depends on the fertilizer you use (most soluble fertilizers lack both Ca and Mg), and whether or not you add lime to soils you make. Most commercial soils will have (dolomitic) lime already added.

Plants need a constant supply of Ca in the nutrient stream for cells to form and form normally, and to prevent BER. There are a number of physical/chemical conditions, other than an actual deficiency of Ca/Mg, that can make these nutrients unavailable.

Al