If the substrate is dry, aerate first ( poke holes) and water to run off, with epsom at  1/2 teaspoon per gal. every three weeks. Mix with a small amount of warm water if possible. Weed loves ES/ magnesium sulfate, real important mineral, but don't over do it ! You can top dress epsom too. If you TD it, use about 1/2 teaspoon per pot. GL