Powder, only for callousing scars if you dont have cinnamon. And you use cinnamon if you dont have or dont want sulfur.

If you want acidity, better add humus, vermicompost or other organic soils in the soil mix.

By add, i mean, use it a small fraction of the soil. The ones listed can be counter productive because of water retention...

I understand used coffee grounds are good for garden plants when placed on top of the soil, like a mulch. By extrapolation I expect it would be good for cactus. But I think using it would be in an experimental arena, so tell us how it does.

I think it could work, I don’t think it would do them harm, as long as you don’t use too much coffee in your water. I have on occasion poured left over tea in my watering can, for the same reason. My great aunt, who was famous for her geraniums, would always give them the last of her tea. There are however cacti that like alkaline conditions, so do your research. I think Ariocarpus is one of those.

Why not use common household white vinegar? Works like a charm for my acid loving containerized plants. I use a capful a gallon. How about using collected rain water?

Apr 17, 2018 - The pH level of a substance lets you know how acidic or basic it is. ... White distilled vinegar, the kind best suited for household cleaning, typically has a pH of around 2.5. ... It is the acetic acid content in vinegar that makes it acidic.

Mar 21, 2017 - We'll tell you which teas are less acidic and why it's safe to keep drinking. ... the type of tea; how you drink tea; how often you drink tea; what you ... But a Turkish study did discover that fruit teas, which tend to be sweet, are more acidic ... Black tea was found to have a pH of 6.37, while coffee had a pH of 5.35 ...

Tea‎: ‎Average pH level
Black‎: ‎4.9-5.5

Mike

But that is the liquids, I bet spent tea leaves and coffee grounds are more nutritous.