Hi, I don't know if it's common to follow up on these posts, but I wanted to give a small update: I've since replanted the cactus following Antti's directions, and it's alive and well, sprouting roots from the damaged part.
So thanks again for helping me save my plant!

What a beautiful collection!
I love your natural arrangements in the pots.

Thank you, Anttisepp!

I'll prop it up as much as possible to keep it out of the soil.

Hello! I’m fairly new to cacti, and I bought this Mammillaria some weeks ago. By browsing this website I learned a bit about soil, so I unpotted it to check the roots and put a more suitable substrate in. Doing this I noticed it had a hole in its side just under the soil line which could fit my fing...