Question
Hello, I would like to ask for your expertise in solving my cactus plant problem.  Today I notice that the lower part of my thick oval-shaped cactus has rotten with yellowish and shrinkage.  I put into some soil and put on a light to shine on it.  Could you tell me what else can I do?  
This type of cactus does not have flowers but the top has the yellowish small spikes.  
Thank you, I hope to hear from your opinion.

Sincerely,
Margaret Lai

Answer
Margaret,

If your plant is a barrel type cactus, there is not much you can do other than what you have done. Typically when a barrel cactus rots, it will not re-root. You might try placing it at an angle so that the bottom that has rotted is exposed to the air. It may root, but it will take a while if it does. Sorry for the bad news. Good luck though!

Sincerely,

Greg