Poorly Cactus

This cactus is just over a foot high and about 25yrs old.After it began to lean;I discovered the roots had completely rotted away and the lower part felt soggy.
After asking on the net, I was advised to cut it cleanly thro' above the deseased area,but, no one told me how to treat the open wound left by the surgery. Surely I do not just plant it straight into compost.
I should be grateful if anyone could help me with this problem. John

 

Let the wound dry off for a few days and when dry plant in well drained gritty cactus compost. Then cross fingers and hope.

 

Also dont bury the bottom of the cutting once it has calloused over.
Just put it on top of a layer of sharp sand, use a few small sticks to keep it upright if needs be, and spray now and again on warm days.
Dont water much, until its firm and has a good root system.

 

I would dust the cut parts with flowers of sulphur. Leave for at least a week to dry off.
And then pot up in a gritty cactus type soil.

If you can provide bottom heat, that will probably improve your chances of rooting it.

 

Thanks to all for your advice