Outside I use pelleted sulfur in the soil, powdered will get there fast but dont use much. Our girls need sulfur and what it does in the soil is mix with O2 and water to make sulfuric acid. You can get this at any garden center.
Peat-moss is also good to mix into the soil for PH down, bananas are acidic also and have good stuff for flowers.
In the pic below, I am mixing soil to go around the plant removing the little ring for the big one. In the bucket is hard wood ash, the sack has egg shells, not in the pit is pelleted dolomite lime and sulfur.