Fertilize them, simple as that. Slow release granules are also good for major nutes. With the granules it's impossible to get lockout or for it to leach out. They usually call it hanging basket fertilizer. If you have the granules and dolomite lime, the only other thing is sulfur and micros. I guess maybe normal lime would be better, then you could use magnesium sulfate (epsum salt) for both magnesium and sulfur.