I have been acidifying 2 gallons of water in watering cans for certain trees and seedlings and they have been honestly explosive this year with perfect healthy growth. They were lagging super hard for the last year till I started doing this. It's a hard job but well, I have a lot more time than usual right now. Also been dosing these same trees with flowering nutrients and it's been working - some of them never flowered before and now are.
I especially acidify my blueberry water, down to 4.0-5.0 since they are acid lovers and again, finally getting gorgeous green growth from them.
If I go too hard on acidifying the water, I see some weird growth that is lime colored on some stuff and it goes dark green again when I get the PH right. Being that my entire collection is in pots due to gopher problems, it's not too bad. If you have a ton of stuff in the ground, there is no way this would work imo and Brad is right with sulphur pellets.