Beginner in NYC

Hello! From Manhattan, New York City. My home greenhouse is a single windowsill on a three-foot wide south-facing window. I have 3 cacti, 1 Mammillaria (not sure what the other 2 are, but not Mammillaria). First: how do I add a photo here so members can help me ID my Mammillaria? Second: i would like advice on best soil mix for it. It is sitting bare-root awaiting a new pot, after I discovered root rot (there is one healthy root left). It is also either dichotomizing or pupping (offsetting), it is not yet clear which way it is going.
I bought this Mammillaria when it was in a juvenile form. In the 2 years of ownership, it has morphed a few times and now looks quite different from its early form.--Saurny

Hi Saurny,

For guidance on how to post photos in a message please look at the "Guidance - How to" section, which has information about just how to do this.

Most Mammillarias tend to prefer a gritty compost. I don't know what proprietary composts you have in NY, but I use 1/3 of a general plant compost (in UK we call it John Innes), 1/3 horticultural grit, and 1/3 volcanic pumice grit. The key, I believe is an open, free draining mix.

PM me if you have any further problems.

Chris