Question
Jeff,

I've been taking care of a friend's cactus for three months now.  Its one of those little round cactus that look like a ball, with fuzzy type needles.  I don't know the actual name.  
I have a plant light set up for it, and its in sandy soil, I water it once or twice a month.  My room is kept rather humid so that may be the problem.
I noticed a few days ago that the top of the cactus has caved in.  And its mushy to the touch.  Its still green and looks fine.  But when you touch it, it feels like a rotten tomato.  Its still firmly planted in the soil, so I haven't tried to look at the roots.  

Any ideas what is wrong?  Is there anything I can do?  Or is it too late?  The soil is dry, but I'm scared to water it for fear it might be some sort of root rot.

Thanks for any help you have.

Answer
Dear Nina,

From what it sounds like, you may have a Notocactus or Mammillaria species.

I suspect that the humidity, combined with inadequate drainage (and possibly temps that it dodn't like, along with inadequate air circulation) did your cactus in. I'm sorry to say that it sounds like a goner.

Cacti can look like they're fine, but what's happened is some sort of internal rot, brought about by the conditions cited above. It is too late. If you can give your next plant more porous soil, air circulation and bright light, it probably would stand a better chance. Many cacti do come from humid environments, but most do not. I suspect yours originates from an area that doesn't like semi-tropical conditions.

Not to worry - even the best growers lose plants.

Hope this helps, and sorry about your plant.

Jeff