Coal slag is a variable byproduct rather than a standardized potting ingredient. Check its source and suitability before adding it around plant roots.
Coal slag is not a standard potting ingredient
Material left from burning coal can vary with fuel and processing. A claim that every batch is clean, sterile or a useful source of nutrients does not provide the information needed to select it for plant roots.
Choose a known growing material
For a container, use a potting mix or amendment sold for horticultural use with its contents identified. Consider particle size, drainage and the needs of the plant. A familiar-looking particle is not enough to establish that two materials behave alike.
Keep feeding separate
A gritty ingredient changes the physical mix; it is not automatically a complete fertilizer. Follow labeled feeding guidance where needed rather than relying on unknown trace elements in a collected byproduct.
Related reading: Perlite in Potting Mixes: Air Space and Drainage.